list.add所有元素都变成最后一个加入的元素

    

public static void main(String[] args) {
    
        List list = new ArrayList<>();
        Test t = new Test();
        for(int i=0;i<10;i++){
            t.setNum(i);
            list.add(t);
        }
    System.out.println(list.toString());
    
    }

因为list添加的是对象的引用，Test只被new了一次然后不断被赋值
    输出

[TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9], TestBean [num=9]]

 

 

	public static void main(String[] args) {
		
		List list = new ArrayList<>();
		
		for(int i=0;i<10;i++){
			Test t = new Test();
			t.setNum(i);
			list.add(t);
		}
	System.out.println(list.toString());
	
	}

 

输出

[TestBean [num=0], TestBean [num=1], TestBean [num=2], TestBean [num=3], TestBean [num=4], TestBean [num=5], TestBean [num=6], TestBean [num=7], TestBean [num=8], TestBean [num=9]]

 

 

 

public class Test {
    private    Integer num;

    public Integer getNum() {
        return num;
    }

    public void setNum(Integer num) {
        this.num = num;
    }

    @Override
    public String toString() {
        return "Test [num=" + num + "]";
    }
    
}