面试官：背了几道面试题就敢说熟悉Java源码？我们不招连源码都不会看的人｜原力计划
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-
你看源码么？ -
你会看源码么？ -
你从源码中有收获么？
String
index
out
of range:
100
public String substring(int beginIndex, int endIndex) {
if (beginIndex <
0) {
//起始坐标小于0
throw
new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex >
value.length) {
//结束坐标大于字符串长度
throw
new StringIndexOutOfBoundsException(endIndex);
}
int subLen = endIndex - beginIndex;
if (subLen <
0) {
//起始坐标大于结束坐标
throw
new StringIndexOutOfBoundsException(subLen);
}
return ((beginIndex ==
0) && (endIndex ==
value.length)) ?
this
:
new String(
value, beginIndex, subLen);
}
if ((beginIndex ==
0) && (endIndex ==
value.length))
return
this;
return
new String(
value, beginIndex, subLen);
3.学习设计模式（针对新手）
import java.util.ResourceBundle;
public
class
Configration {
private
static Object
lock =
new Object();
private
static Configration config =
null;
private
static ResourceBundle rb =
null;
private Configration(String filename) {
rb = ResourceBundle.getBundle(filename);
}
public static Configration getInstance(String filename) {
synchronized(
lock) {
if(
null == config) {
config =
new Configration(filename);
}
}
return (config);
}
public String getValue(String key) {
String ret =
"";
if(rb.containsKey(key))
{
ret = rb.getString(key);
}
return ret;
}
}
public
class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
//序列号
private static final long serialVersionUID = 362498820763181265L;
/**
* 初始容量，必须是2的幂数
* 1 << 4 = 10000 = 16
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 初始默认值二进制1左移四位 = 16
/**
* 最大容量
* 必须是2的幂数 <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* 加载因子，构造函数中没有指定时会被使用
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 从链表转到树的时机
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* 从树转到链表的时机
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
static final int MIN_TREEIFY_CAPACITY = 64;
这样，我们就对HashMap中常量的作用和意义有所理解了
/**
* 构造一个空的，带有初始值和初始加载因子的HashMap
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity <
0)
throw
new IllegalArgumentException(
"Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <=
0 || Float.isNaN(loadFactor))
throw
new IllegalArgumentException(
"Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
static final int tableSizeFor(int cap) {
int n = cap -
1;
n |= n >>>
1;
n |= n >>>
2;
n |= n >>>
4;
n |= n >>>
8;
n |= n >>>
16;
return (n <
0) ?
1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n +
1;
}
public V put(K key, V value) {
return putVal(hash(key), key,
value,
false,
true);
}
static final int hash(Object key) {
int h;
return (key ==
null) ?
0 : (h = key.hashCode()) ^ (h >>>
16);
}
/**
* Implements Map.put and related methods.
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p;
int n, i;
if ((tab = table) ==
null || (n = tab.length) ==
0)
n = (tab = resize()).length;
if ((p = tab[i = (n -
1) & hash]) ==
null)
tab[i] = newNode(hash, key,
value,
null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key !=
null && key.
equals(k))))
e = p;
else
if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(
this, tab, hash, key,
value);
else {
for (
int binCount =
0; ; ++binCount) {
if ((e = p.next) ==
null) {
p.next = newNode(hash, key,
value,
null);
if (binCount >= TREEIFY_THRESHOLD -
1)
// -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key !=
null && key.
equals(k))))
break;
p = e;
}
}
if (e !=
null) {
// existing mapping for key
V oldValue = e.
value;
if (!onlyIfAbsent || oldValue ==
null)
e.
value =
value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return
null;
}
/**
* 继承于 Map.put.
*
* @param hash
key的hash值
* @param
key
key
* @param value 要输入的值
* @param onlyIfAbsent 如果是
true, 不改变存在的值
* @param evict
if
false, the table
is
in creation mode.
* @
return 返回当前值, 当前值不存在返回null
*/
static
class Node<K,V> implements Map.Entry<K,V> {
final
int hash;
final K key;
V value;
Node<K,V> next;
Node(
int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
}
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