为什么可以在for ... each循环中修改自定义对象的ArrayList

Question

我们有一个简单的自定义对象:

public class CustomObject {
    public CustomObject(int myIntNumber, String myString) {
    this.myIntNumber = myIntNumber ;
    this.myString = myString;    
    }

    private int myIntNumber;
    private String myString;

    public void setMyIntNumber(int myIntNumber) {
    this.myIntNumber = myIntNumber;
}

public int getMyIntNumber() {
    return myIntNumber;
}

public void setMyString(String myString) {
    this.myString = myString;
}

public String getMyString() {
    return myString;
}


    public String toString() {
        return "CustomObject [" + String.valueOf(myIntNumber)  + ", "+ myString+"]" ;  
    }
}

并且,我们尝试使用for ... each循环修改此类对象的ArrayList.当无法以这种方式修改String对象或Integer对象的ArrayList时,为什么列表中的对象会被修改？

我的测试代码:

import java.util.ArrayList;

public class TestTraveringListModification {

    public static void main(String[] args) {

    ArrayList<String> sList  = new ArrayList<String>();
    sList.add("String a");
    sList.add("String b");
    sList.add("C");
    sList.add("D");
    sList.add("String f");
    sList.add("String e");

    System.out.println("Before: "+sList);
    for (String s : sList) {
        s="asdf" ;

    }

    System.out.println("After: "+ sList);

    ArrayList<CustomObject> objL = new ArrayList<CustomObject> () ;
    objL.add(new CustomObject (1, "test") );
    objL.add(new CustomObject (2, "jim") );
    objL.add(new CustomObject (20, "dec") );
    objL.add(new CustomObject (60, "what") );
  System.out.println("before: "+ objL );
    for(CustomObject co : objL ){    
        co.setMyIntNumber(-1);
        co.setMyString("modified String");
    }
    System.out.println("after: "+objL);


    ArrayList<Integer> numList = new ArrayList<Integer>(); 

    numList.add(1);
    numList.add(3);
    numList.add(5);
    numList.add(67);
    numList.add(9598);

    System.out.println("before: "+ numList);

    for (Integer i : numList){
        i = 8; 
    }
    System.out.println("after: "+ numList);

}
}

运行它将产生以下输出:

Before: [String a, String b, C, D, String f, String e]

After: [String a, String b, C, D, String f, String e]

before: [CustomObject [1, test], CustomObject [2, jim], CustomObject [20, dec], CustomObject [60, what]]

after: [CustomObject [-1, modified String], CustomObject [-1, modified String], CustomObject [-1, modified String], CustomObject [-1, modified String]]

before: [1, 3, 5, 67, 9598]

• 项目清单

  after: [1, 3, 5, 67, 9598]

那么,为什么我可以修改objL而不是sList或numList？

Answer 1

因为重新分配和变异是两回事.

s = "asdf"会改变所指的s内容.它曾经包含对成员的引用sList,现在它指的是"asdf".这一变化与该成员无关sList.

与同类i和numList,虽然不是完全完全相同.numList包含Integer对象,从autoboxed 1,3,5... for将分配Integer对象的值i.如果您随后将值更改i为(autoboxed)Integer(8),则它也不会影响numList任何值.

然而,有了co和objL,你做了一件非常不同的事情.您没有重新分配co给另一个对象(不会影响对象objL),而是选择调用方法co,然后恰好改变它们的状态.请注意,这里您没有修改objL,您正在修改它包含的对象.

关键的见解是i,co并且s不是相应列表的元素.它们包含的值是可能是数组中的元素(但再次autounboxing的情况下,这也太不成立).