java8List.sort()排序常用方法
Posted 沛沛老爹
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常规操作
升序
普通升序
list.sort(Comparator.comparing(T::getSort))多条件升序
先按照排序字段排序，如果排序字段值相等，则继续比较创建时间。
list.sort(Comparator.comparing(T::getSort).thenComparing(T::getCreatTime));降序
使用reversed的方法进行反向排序
list.sort(Comparator.comparing(T::getSort).reversed());升序+降序
list.sort(Comparator.comparing(T::getSort)
.thenComparing(T::getCreateTime).reversed());
对象排序
内比较器-Comparable
Comparable在集合内部定义的方法实现的排序，位于java.lang下。Comparable支持自比较，自比较是指比如String等类里面本身就有CompareTo()方法，直接就可以进行String类对象的比较，也就是说，实现了Comparable接口的类的对象的列表或数组可以通过Collections.sort或Arrays.sort进行自动排序，也就是大家常说的自然排序
Comparable的源码
public interface Comparable<T>
/**
* Compares this object with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* <p>The implementor must ensure
* @code sgn(x.compareTo(y)) == -sgn(y.compareTo(x))
* for all @code x and @code y. (This
* implies that @code x.compareTo(y) must throw an exception iff
* @code y.compareTo(x) throws an exception.)
*
* <p>The implementor must also ensure that the relation is transitive:
* @code (x.compareTo(y) > 0 && y.compareTo(z) > 0) implies
* @code x.compareTo(z) > 0.
*
* <p>Finally, the implementor must ensure that @code x.compareTo(y)==0
* implies that @code sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for
* all @code z.
*
* <p>It is strongly recommended, but <i>not</i> strictly required that
* @code (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any
* class that implements the @code Comparable interface and violates
* this condition should clearly indicate this fact. The recommended
* language is "Note: this class has a natural ordering that is
* inconsistent with equals."
*
* <p>In the foregoing description, the notation
* @code sgn(<i>expression</i>@code ) designates the mathematical
* <i>signum</i> function, which is defined to return one of @code -1,
* @code 0, or @code 1 according to whether the value of
* <i>expression</i> is negative, zero, or positive, respectively.
*
* @param o the object to be compared.
* @return a negative integer, zero, or a positive integer as this object
* is less than, equal to, or greater than the specified object.
*
* @throws NullPointerException if the specified object is null
* @throws ClassCastException if the specified object's type prevents it
* from being compared to this object.
*/
public int compareTo(T o);
整个内部只有一个接口方法：public int compareTo(T o);
它的作用主要是比较当前实现接口的对象与指定对象o的顺序。它根据比较的结果返回负数、0和正数。
相对应的关系如下：
1、如果 o1.compareTo(o2)<0 ==> o1<o2
2、o1.compareTo(o2)==0 ==> o1==o2
3、o1.compareTo(o2)>0 ==> o1>o2
实现方式
常规的实现方式，是类实现Comparable接口和实现对应的compareTo(T o) 方法。
假设我们要实现对User对象的比较。排序条件是根据用户年纪age的大小来区分。
示例代码如下
1、让User类实现Comparable接口
2、重新compareTo方法，使用当前对象的age-参数的年纪，得到返回结果来进行判定。
@Data
public class User implements Comparable<User>
/**姓名*/
private String name;
/**年纪*/
private Integer age;
/**
* 重写compareTo方法，根据年纪进行排序
*/
@Override
public int compareTo(User u)
return this.age-u.getAge();
测试验证：
public static void main(String[] args)
User[] userArr=new User[]new User("Zhangsan", 20),new User("Lisi", 10);
System.out.println("======排序前======");
for (User user : userArr)
System.out.print(user.getName()+":"+user.getAge());
System.out.println("=======排序后======");
Arrays.sort(userArr);
for (User user : userArr)
System.out.print(user.getName()+":"+user.getAge());
测试结果就不输出了，您可以复制代码自己试下看看。
说到这里了，大家有可能就会问了。如果我有已经写好的类了，没有实现Comparable接口，但是我不想去改它了。还可以实现排序么？
当然是可以的，这个时候，我们的Comparator就可以拿出来比划比划了。。。
外比较器-Comparator
Comparator是在集合外部实现的排序，位于java.util下。它
源代码（代码有点长，建议跳过该步骤）
/*
* Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.util;
import java.io.Serializable;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.ToDoubleFunction;
import java.util.Comparators;
/**
* A comparison function, which imposes a <i>total ordering</i> on some
* collection of objects. Comparators can be passed to a sort method (such
* as @link Collections#sort(List,Comparator) Collections.sort or @link
* Arrays#sort(Object[],Comparator) Arrays.sort) to allow precise control
* over the sort order. Comparators can also be used to control the order of
* certain data structures (such as @link SortedSet sorted sets or @link
* SortedMap sorted maps), or to provide an ordering for collections of
* objects that don't have a @link Comparable natural ordering.<p>
*
* The ordering imposed by a comparator @code c on a set of elements
* @code S is said to be <i>consistent with equals</i> if and only if
* @code c.compare(e1, e2)==0 has the same boolean value as
* @code e1.equals(e2) for every @code e1 and @code e2 in
* @code S.<p>
*
* Caution should be exercised when using a comparator capable of imposing an
* ordering inconsistent with equals to order a sorted set (or sorted map).
* Suppose a sorted set (or sorted map) with an explicit comparator @code c
* is used with elements (or keys) drawn from a set @code S. If the
* ordering imposed by @code c on @code S is inconsistent with equals,
* the sorted set (or sorted map) will behave "strangely." In particular the
* sorted set (or sorted map) will violate the general contract for set (or
* map), which is defined in terms of @code equals.<p>
*
* For example, suppose one adds two elements @code a and @code b such that
* @code (a.equals(b) && c.compare(a, b) != 0)
* to an empty @code TreeSet with comparator @code c.
* The second @code add operation will return
* true (and the size of the tree set will increase) because @code a and
* @code b are not equivalent from the tree set's perspective, even though
* this is contrary to the specification of the
* @link Set#add Set.add method.<p>
*
* Note: It is generally a good idea for comparators to also implement
* @code java.io.Serializable, as they may be used as ordering methods in
* serializable data structures (like @link TreeSet, @link TreeMap). In
* order for the data structure to serialize successfully, the comparator (if
* provided) must implement @code Serializable.<p>
*
* For the mathematically inclined, the <i>relation</i> that defines the
* <i>imposed ordering</i> that a given comparator @code c imposes on a
* given set of objects @code S is:<pre>
* (x, y) such that c.compare(x, y) <= 0.
* </pre> The <i>quotient</i> for this total order is:<pre>
* (x, y) such that c.compare(x, y) == 0.
* </pre>
*
* It follows immediately from the contract for @code compare that the
* quotient is an <i>equivalence relation</i> on @code S, and that the
* imposed ordering is a <i>total order</i> on @code S. When we say that
* the ordering imposed by @code c on @code S is <i>consistent with
* equals</i>, we mean that the quotient for the ordering is the equivalence
* relation defined by the objects' @link Object#equals(Object)
* equals(Object) method(s):<pre>
* (x, y) such that x.equals(y). </pre>
*
* <p>Unlike @code Comparable, a comparator may optionally permit
* comparison of null arguments, while maintaining the requirements for
* an equivalence relation.
*
* <p>This interface is a member of the
* <a href="@docRoot/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
*
* @param <T> the type of objects that may be compared by this comparator
*
* @author Josh Bloch
* @author Neal Gafter
* @see Comparable
* @see java.io.Serializable
* @since 1.2
*/
@FunctionalInterface
public interface Comparator<T>
/**
* Compares its two arguments for order. Returns a negative integer,
* zero, or a positive integer as the first argument is less than, equal
* to, or greater than the second.<p>
*
* The implementor must ensure that @code sgn(compare(x, y)) ==
* -sgn(compare(y, x)) for all @code x and @code y. (This
* implies that @code compare(x, y) must throw an exception if and only
* if @code compare(y, x) throws an exception.)<p>
*
* The implementor must also ensure that the relation is transitive:
* @code ((compare(x, y)>0) && (compare(y, z)>0)) implies
* @code compare(x, z)>0.<p>
*
* Finally, the implementor must ensure that @code compare(x, y)==0
* implies that @code sgn(compare(x, z))==sgn(compare(y, z)) for all
* @code z.<p>
*
* It is generally the case, but <i>not</i> strictly required that
* @code (compare(x, y)==0) == (x.equals(y)). Generally speaking,
* any comparator that violates this condition should clearly indicate
* this fact. The recommended language is "Note: this comparator
* imposes orderings that are inconsistent with equals."<p>
*
* In the foregoing description, the notation
* @code sgn(<i>expression</i>@code ) designates the mathematical
* <i>signum</i> function, which is defined to return one of @code -1,
* @code 0, or @code 1 according to whether the value of
* <i>expression</i> is negative, zero, or positive, respectively.
*
* @param o1 the first object to be compared.
* @param o2 the second object to be compared.
* @return a negative integer, zero, or a positive integer as the
* first argument is less than, equal to, or greater than the
* second.
* @throws NullPointerException if an argument is null and this
* comparator does not permit null arguments
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this comparator.
*/
int compare(T o1, T o2);
/**
* Indicates whether some other object is "equal to" this
* comparator. This method must obey the general contract of
* @link Object#equals(Object). Additionally, this method can return
* @code true <i>only</i> if the specified object is also a comparator
* and it imposes the same ordering as this comparator. Thus,
* @code comp1.equals(comp2) implies that @code sgn(comp1.compare(o1,
* o2))==sgn(comp2.compare(o1, o2)) for every object reference
* @code o1 and @code o2.<p>
*
* Note that it is <i>always</i> safe <i>not</i> to override
* @code Object.equals(Object). However, overriding this method may,
* in some cases, improve performance by allowing programs to determine
* that two distinct comparators impose the same order.
*
* @param obj the reference object with which to compare.
* @return @code true only if the specified object is also
* a comparator and it imposes the same ordering as this
* comparator.
* @see Object#equals(Object)
* @see Object#hashCode()
*/
boolean equals(Object obj);
/**
* Returns a comparator that imposes the reverse ordering of this
* comparator.
*
* @return a comparator that imposes the reverse ordering of this
* comparator.
* @since 1.8
*/
default Comparator<T> reversed()
return Collections.reverseOrder(this);
/**
* Returns a lexicographic-order comparator with another comparator.
* If this @code Comparator considers two elements equal, i.e.
* @code compare(a, b) == 0, @code other is used to determine the order.
*
* <p>The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @apiNote
* For example, to sort a collection of @code String based on the length
* and then case-insensitive natural ordering, the comparator can be
* composed using following code,
*
* <pre>@code
* Comparator<String> cmp = Comparator.comparingInt(String::length)
* .thenComparing(String.CASE_INSENSITIVE_ORDER);
* </pre>
*
* @param other the other comparator to be used when this comparator
* compares two objects that are equal.
* @return a lexicographic-order comparator composed of this and then the
* other comparator
* @throws NullPointerException if the argument is null.
* @since 1.8
*/
default Comparator<T> thenComparing(Comparator<? super T> other)
Objects.requireNonNull(other);
return (Comparator<T> & Serializable) (c1, c2) ->
int res = compare(c1, c2);
return (res != 0) ? res : other.compare(c1, c2);
;
/**
* Returns a lexicographic-order comparator with a function that
* extracts a key to be compared with the given @code Comparator.
*
* @implSpec This default implementation behaves as if @code
* thenComparing(comparing(keyExtractor, cmp)).
*
* @param <U> the type of the sort key
* @param keyExtractor the function used to extract the sort key
* @param keyComparator the @code Comparator used to compare the sort key
* @return a lexicographic-order comparator composed of this comparator
* and then comparing on the key extracted by the keyExtractor function
* @throws NullPointerException if either argument is null.
* @see #comparing(Function, Comparator)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default <U> Comparator<T> thenComparing(
Function<? super T, ? extends U> keyExtractor,
Comparator<? super U> keyComparator)
return thenComparing(comparing(keyExtractor, keyComparator));
/**
* Returns a lexicographic-order comparator with a function that
* extracts a @code Comparable sort key.
*
* @implSpec This default implementation behaves as if @code
* thenComparing(comparing(keyExtractor)).
*
* @param <U> the type of the @link Comparable sort key
* @param keyExtractor the function used to extract the @link
* Comparable sort key
* @return a lexicographic-order comparator composed of this and then the
* @link Comparable sort key.
* @throws NullPointerException if the argument is null.
* @see #comparing(Function)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default <U extends Comparable<? super U>> Comparator<T> thenComparing(
Function<? super T, ? extends U> keyExtractor)
return thenComparing(comparing(keyExtractor));
/**
* Returns a lexicographic-order comparator with a function that
* extracts an @code int sort key.
*
* @implSpec This default implementation behaves as if @code
* thenComparing(comparingInt(keyExtractor)).
*
* @param keyExtractor the function used to extract the integer sort key
* @return a lexicographic-order comparator composed of this and then the
* @code int sort key
* @throws NullPointerException if the argument is null.
* @see #comparingInt(ToIntFunction)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor)
return thenComparing(comparingInt(keyExtractor));
/**
* Returns a lexicographic-order comparator with a function that
* extracts a @code long sort key.
*
* @implSpec This default implementation behaves as if @code
* thenComparing(comparingLong(keyExtractor)).
*
* @param keyExtractor the function used to extract the long sort key
* @return a lexicographic-order comparator composed of this and then the
* @code long sort key
* @throws NullPointerException if the argument is null.
* @see #comparingLong(ToLongFunction)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor)
return thenComparing(comparingLong(keyExtractor));
/**
* Returns a lexicographic-order comparator with a function that
* extracts a @code double sort key.
*
* @implSpec This default implementation behaves as if @code
* thenComparing(comparingDouble(keyExtractor)).
*
* @param keyExtractor the function used to extract the double sort key
* @return a lexicographic-order comparator composed of this and then the
* @code double sort key
* @throws NullPointerException if the argument is null.
* @see #comparingDouble(ToDoubleFunction)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
return thenComparing(comparingDouble(keyExtractor));
/**
* Returns a comparator that imposes the reverse of the <em>natural
* ordering</em>.
*
* <p>The returned comparator is serializable and throws @link
* NullPointerException when comparing @code null.
*
* @param <T> the @link Comparable type of element to be compared
* @return a comparator that imposes the reverse of the <i>natural
* ordering</i> on @code Comparable objects.
* @see Comparable
* @since 1.8
*/
public static <T extends Comparable<? super T>> Comparator<T> reverseOrder()
return Collections.reverseOrder();
/**
* Returns a comparator that compares @link Comparable objects in natural
* order.
*
* <p>The returned comparator is serializable and throws @link
* NullPointerException when comparing @code null.
*
* @param <T> the @link Comparable type of element to be compared
* @return a comparator that imposes the <i>natural ordering</i> on @code
* Comparable objects.
* @see Comparable
* @since 1.8
*/
@SuppressWarnings("unchecked")
public static <T extends Comparable<? super T>> Comparator<T> naturalOrder()
return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
/**
* Returns a null-friendly comparator that considers @code null to be
* less than non-null. When both are @code null, they are considered
* equal. If both are non-null, the specified @code Comparator is used
* to determine the order. If the specified comparator is @code null,
* then the returned comparator considers all non-null values to be equal.
*
* <p>The returned comparator is serializable if the specified comparator
* is serializable.
*
* @param <T> the type of the elements to be compared
* @param comparator a @code Comparator for comparing non-null values
* @return a comparator that considers @code null to be less than
* non-null, and compares non-null objects with the supplied
* @code Comparator.
* @since 1.8
*/
public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)
return new Comparators.NullComparator<>(true, comparator);
/**
* Returns a null-friendly comparator that considers @code null to be
* greater than non-null. When both are @code null, they are considered
* equal. If both are non-null, the specified @code Comparator is used
* to determine the order. If the specified comparator is @code null,
* then the returned comparator considers all non-null values to be equal.
*
* <p>The returned comparator is serializable if the specified comparator
* is serializable.
*
* @param <T> the type of the elements to be compared
* @param comparator a @code Comparator for comparing non-null values
* @return a comparator that considers @code null to be greater than
* non-null, and compares non-null objects with the supplied
* @code Comparator.
* @since 1.8
*/
public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)
return new Comparators.NullComparator<>(false, comparator);
/**
* Accepts a function that extracts a sort key from a type @code T, and
* returns a @code Comparator<T> that compares by that sort key using
* the specified @link Comparator.
*
* <p>The returned comparator is serializable if the specified function
* and comparator are both serializable.
*
* @apiNote
* For example, to obtain a @code Comparator that compares @code
* Person objects by their last name ignoring case differences,
*
* <pre>@code
* Comparator<Person> cmp = Comparator.comparing(
* Person::getLastName,
* String.CASE_INSENSITIVE_ORDER);
* </pre>
*
* @param <T> the type of element to be compared
* @param <U> the type of the sort key
* @param keyExtractor the function used to extract the sort key
* @param keyComparator the @code Comparator used to compare the sort key
* @return a comparator that compares by an extracted key using the
* specified @code Comparator
* @throws NullPointerException if either argument is null
* @since 1.8
*/
public static <T, U> Comparator<T> comparing(
Function<? super T, ? extends U> keyExtractor,
Comparator<? super U> keyComparator)
Objects.requireNonNull(keyExtractor);
Objects.requireNonNull(keyComparator);
return (Comparator<T> & Serializable)
(c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
keyExtractor.apply(c2));
/**
* Accepts a function that extracts a @link java.lang.Comparable
* Comparable sort key from a type @code T, and returns a @code
* Comparator<T> that compares by that sort key.
*
* <p>The returned comparator is serializable if the specified function
* is also serializable.
*
* @apiNote
* For example, to obtain a @code Comparator that compares @code
* Person objects by their last name,
*
* <pre>@code
* Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
* </pre>
*
* @param <T> the type of element to be compared
* @param <U> the type of the @code Comparable sort key
* @param keyExtractor the function used to extract the @link
* Comparable sort key
* @return a comparator that compares by an extracted key
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
Function<? super T, ? extends U> keyExtractor)
Objects.requireNonNull(keyExtractor);
return (Comparator<T> & Serializable)
(c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
/**
* Accepts a function that extracts an @code int sort key from a type
* @code T, and returns a @code Comparator<T> that compares by that
* sort key.
*
* <p>The returned comparator is serializable if the specified function
* is also serializable.
*
* @param <T> the type of element to be compared
* @param keyExtractor the function used to extract the integer sort key
* @return a comparator that compares by an extracted key
* @see #comparing(Function)
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor)
Objects.requireNonNull(keyExtractor);
return (Comparator<T> & Serializable)
(c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
/**
* Accepts a function that extracts a @code long sort key from a type
* @code T, and returns a @code Comparator<T> that compares by that
* sort key.
*
* <p>The returned comparator is serializable if the specified function is
* also serializable.
*
* @param <T> the type of element to be compared
* @param keyExtractor the function used to extract the long sort key
* @return a comparator that compares by an extracted key
* @see #comparing(Function)
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor)
Objects.requireNonNull(keyExtractor);
return (Comparator<T> & Serializable)
(c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
/**
* Accepts a function that extracts a @code double sort key from a type
* @code T, and returns a @code Comparator<T> that compares by that
* sort key.
*
* <p>The returned comparator is serializable if the specified function
* is also serializable.
*
* @param <T> the type of element to be compared
* @param keyExtractor the function used to extract the double sort key
* @return a comparator that compares by an extracted key
* @see #comparing(Function)
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static<T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor)
Objects.requireNonNull(keyExtractor);
return (Comparator<T> & Serializable)
(c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
方法有点多，这里就简单说下要实现的方法。int compare(T o1, T o2)，此方法是比较o1和o2的大小。返回的和compareTo的结果方式差不多
1、如果 compare(o1,o2)<0 ==> o1<o2
2、compare(o1,o2)==0 ==> o1==o2
3、compare(o1,o2)>0 ==> o1>o2
假如我们的User类，我们不想修改原来的代码了。我们可以按照下面的思路来解决：
1、创建一个新的类，让它实现Comparator接口。然后sort的时候使用其来作为比较器使用。
代码
1、实体类
@Data
public class User
/**姓名*/
private String name;
/**年纪*/
private Integer age;
/**
* 重写compareTo方法，根据年纪进行排序
*/a
@Override
public int compareTo(User u)
return this.age-u.getAge();
2、比较器
public class UserCompartor implements Comparator<User>
@Override
public int compare(User o1, User o2)
return o1.getAge()-o2.getAge();
3、验证代码
public static void main(String[] args)
User[] userArr=new User[]new User("Zhangsan", 20),new User("Lisi", 10);
System.out.println("======排序前======");
for (User user : userArr)
System.out.print(user.getName()+":"+user.getAge());
System.out.println("=======排序后======");
Arrays.sort(userArr,new UserCompartor());
for (User user : userArr)
System.out.print(user.getName()+":"+user.getAge());
总结
Java 中的两种对象排序方式：
- Comparable 自然排序。（由实体类实现）
- Comparator 是定制排序。（无法修改实体类时，直接在调用方创建）
到了这里，大家是否有个疑问？为什么前面简单的String,Integer这些可以直接使用sort的排序来实现，而自定义的对象却需要去重写对应的方法？
这是因为，普通的数据类型它们默认实现了Comparable 接口，实现了 compareTo 方法，所以我们可以直接使用。不信的话，你可以查找对应的基础数据类型的类看下。
下面Integer的这个大家可以看下
package java.lang;
import java.lang.annotation.Native;
import java.util.Objects;
import jdk.internal.HotSpotIntrinsicCandidate;
import jdk.internal.misc.VM;
import static java.lang.String.COMPACT_STRINGS;
import static java.lang.String.LATIN1;
import static java.lang.String.UTF16;
/**
* The @code Integer class wraps a value of the primitive type
* @code int in an object. An object of type @code Integer
* contains a single field whose type is @code int.
*
* <p>In addition, this class provides several methods for converting
* an @code int to a @code String and a @code String to an
* @code int, as well as other constants and methods useful when
* dealing with an @code int.
*
* <p>Implementation note: The implementations of the "bit twiddling"
* methods (such as @link #highestOneBit(int) highestOneBit and
* @link #numberOfTrailingZeros(int) numberOfTrailingZeros) are
* based on material from Henry S. Warren, Jr.'s <i>Hacker's
* Delight</i>, (Addison Wesley, 2002).
*
* @author Lee Boynton
* @author Arthur van Hoff
* @author Josh Bloch
* @author Joseph D. Darcy
* @since 1.0
*/
public final class Integer extends Number implements Comparable<Integer> 以上是关于java8List.sort()排序常用方法的主要内容，如果未能解决你的问题，请参考以下文章
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