List排序进阶

Posted 2020-11-17 fengyumeng

初始代码：

一个Apple类

public class Apple{
    String color;
    Integer size;
    Float weight;
    
    public Apple() {
    }

    public Apple(String color, Integer size, Float weight) {
        this.color = color;
        this.size = size;
        this.weight = weight;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public Integer getSize() {
        return size;
    }

    public void setSize(Integer size) {
        this.size = size;
    }

    public Float getWeight() {
        return weight;
    }

    public void setWeight(Float weight) {
        this.weight = weight;
    }

    @Override
    public String toString() {
        return "Apple{" +
                "weight=" + weight +
                ", size=" + size +
                ", color=" + color +
                ‘}‘;
    }
}

View Code

 

主函数 一个赋值一个打印

    public static void main(String[] args) {
        List<Apple> appleList = new ArrayList<>();
        appleList.add(new Apple("red",111,333F));
        appleList.add(new Apple("yellow",333,222F));
        appleList.add(new Apple("blue",222,888F));
        appleList.add(new Apple("black",555,777F));
        appleList.add(new Apple("pink",444,111F));
        appleList.add(new Apple("green",666,444F));
    }

    private static <T> void forEach(List<T> list){
        for(Object vo : list){
            System.out.println(vo.toString());
        }
    }

 

第一级：传递代码

Apple类实现Comparator 复写compare

public class Apple implements Comparator<Apple> {
    String color;
    Integer size;
    Float weight;

    @Override
    public int compare(Apple o1, Apple o2) {
        return o1.getSize().compareTo(o2.getSize());
    }
    
    ……
}

        appleList.sort(new Apple());
        forEach(appleList);

打印结果：

 

第二级：匿名类

Apple类还原 不用实现Comparator

        appleList.sort(new Comparator<Apple>() {
            @Override
            public int compare(Apple o1, Apple o2) {
                return o1.getSize().compareTo(o2.getSize());
            }
        });
        forEach(appleList);

打印结果一样

 

第三级：使用Lambda表达式

一样的传递代码 轻量的写法

        appleList.sort((Apple o1,Apple o2) -> o1.getSize().compareTo(o2.getSize()));
        forEach(appleList);

 

能再简洁点吗？  能。

java编译器可以根据lambda的上下文来推断表达式参数的类型，所以写Lambda的时候可以省略参数类型

        appleList.sort((o1,o2) -> o1.getSize().compareTo(o2.getSize()));
        forEach(appleList);

 

还能再简洁点吗？ 能。

Comparator有一个comparing的静态辅助方法，可以接受一个函数来提取Comparable的键值，并生成一个Comparator对象

        appleList.sort(Comparator.comparing((a)->a.getSize()));
        forEach(appleList);

 

第四级：使用方法引用

        appleList.sort(Comparator.comparing(Apple::getSize));
        forEach(appleList);

方法引用仅仅是涉及单一方法的Lampbda语法糖，使用方式和php中的调用静态方法一样，类::方法。