调用 .clear() 后，如何在 Java 中备份 ArrayList?

问题描述

我正在用 Java 编程，我想备份 ArrayList在我清理它之前不幸的是，在我调用 clear() 之后，备份也被清理了.我该如何解决?

I'm programming in Java and I would like to take a backup of an ArrayListbefore I clean itUnfortunately, after I call clear(), the back up is cleaned as well.How can I solve that?

我的程序如下:

public class Main {

public static void main(String args[]) {

    ArrayListMethod ALM = new ArrayListMethod();
    ALM.FillArrayList();

}

}

import java.util.ArrayList;

public class ArrayListMethod {

public void FillArrayList() {

    ArrayList AL;
    AL =  new ArrayList();
    ArrayList BACKUPArrayList = new ArrayList();

    for(int i = 0 ; i <= 5; i++) {
        AL.add(i);
        BACKUPArrayList = AL;
        System.out.println(" ARRAY LIST BEFORE CLEAR " + AL);
        AL.clear();
    }
    System.out.println(" BACK UP LIST AFTER CLEAR" + BACKUPArrayList);
    //Unfortunately the BACKUPArrayList is cleaned as well...

}

}

推荐答案

你的备份列表也被清除的原因是在 java 中所有变量都是引用.这意味着 BACKUPArrayList = AL; 行不会将 AL 的内容复制到 BACKUPArrayList 而是使变量(不是对象！)BACKUPArrayList 引用 AL.所以 AL 和 BACKUPArrayList 在分配之后引用同一个列表.

The reason why your backup list is cleared as well is that in java all variables are references. This means that the line BACKUPArrayList = AL; doesnt copy the contents of AL to BACKUPArrayList but makes the variable (not object!) BACKUPArrayList references AL. So both AL and BACKUPArrayList reference the same list after that assignment.

要解决这个问题，只需调用BACKUPArrayList.addAll(AL);.这一行将把 AL 的所有元素添加到 BACKUPArrayList.另请参阅:http://docs.oracle.com/javase/7/docs/api/java/util/AbstractList.html

To solve this simply call BACKUPArrayList.addAll(AL);. This line will add all elements of AL to BACKUPArrayList. See also: http://docs.oracle.com/javase/7/docs/api/java/util/AbstractList.html

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