java - 通过引用传递树桩新手

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 那么为什么这个子类会为 aList[0] 返回错误的值呢？当我希望每个类都修改同一个数组时，似乎每个类都在修改自己的数组。

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();

在这里，您将创建两个新对象，因此它们将拥有自己的列表，并且不会像您期望的那样共享一个列表。

您正在寻找的是:

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

试试这个..