Question

根据javadocs“子类继承所有成员（字段，方法和嵌套类）from its superclass”。另外，Java manipulates objects by 引用为什么这个子类为aList [0]返回了错误的值？当我希望它们都修改同一个数组时，似乎每个类都在修改它们自己的数组。

public class mystery {
protected List<String> aList;

public mystery() {
    aList = new ArrayList<String>();
}

public void addToArray() {
    //"foo" is successfully added to the arraylist
    aList.add("foo");
}

public void printArray() {
    System.out.println( "printArray " + aList.get(0) +"" );
}
public static void main(String[] args) {
    mystery prob1 = new mystery();
    mysterySubclass prob2 = new mysterySubclass();
    //add "foo" to array
    prob1.addToArray();
    //add "bar" to array
    prob2.addToArray2();

    //expect to print "foo", works as expected
    prob1.printArray();
    //expect to print "foo", but actually prints "bar"
    prob2.printArray();
    //expect to print "foo", but actually prints "bar"
    prob2.printArray2();
}

}

public class mysterySubclass extends mystery {

public void mysterySubclass() {}
public void addToArray2() {
    aList.add("bar");
}

public void printArray2() {
    System.out.println( "printArray2 " + super.aList.get(0) +"" );
}

}

Answer 1

    mystery prob1 = new mystery();
    mysterySubclass prob2 = new mysterySubclass();

您正在创建两个新对象，因此他们将拥有自己的列表，并且不会按预期分享一个。

您正在寻找的是：

    mystery prob1 = new mysterySubclass();
    mysterySubclass prob2 = (mysterySubclass)prob1;

试试这个..

Answer 2

这是因为您正在调用prob2.addToArray2();，这会将"bar"字符串添加到aList变量中的prob2属性中。考虑到它们共享相同的属性，而不是对象引用的相同引用，因此每个prob和prob2都有自己的aList属性值。使用调试器可以很容易地证明这种行为。

Answer 3

mystery prob1 = new mystery();
mysterySubclass prob2 = new mysterySubclass();
//add "foo" to array
prob1.addToArray();
//add "bar" to array
prob2.addToArray2();

//expect to print "foo", works as expected
prob1.printArray();
//expect to print "foo", but actually prints "bar"
prob2.printArray();
//expect to print "foo", but actually prints "bar"
prob2.printArray2();

你的prob1有一个列表你的prob2有一个列表是的，他们是不同的。

在第4行=＆gt;之后prob1.aList - foo

第6行后的

=＆gt; prob2.aList - bar

所以prob2.printArray（）只有“bar”来打印，还有prob2.printArray2（）基本上他们做同样的事情。

通过参考树桩新手

3 个答案: