用subClass toString - java打印subClass数组列表

Question

超类中有一个toString，子类中有一个toString。我想在超类的ArrayList中显示东西，但使用子类的toString。例如：

public class SuperClass{
    // field
    String name;
    // methods 
    public SuperClass(String name) {
        this.name = name;
    }
    @override
    public String toString() {
        return "Name: " + name;
    }
}

那么我有一个子类

public class SubClass extends SuperClass {
    // field 
    private int age;
    // methods
    public SubClass(String name, int age) {
        super(name);
        this.age = age;
    }
    @override
    public String toString() {
        return "Name: " + name +
               "\nAge: " + age;
    }
}

我想用子类的MyList在类toString中显示这个超类。，这有可能吗？

import java.util.ArrayList;

public class MyList {
    protected ArrayList<SuperClass> list = new ArrayList<>(); // display with Name and Age

    public void userInput() {
        // user inputting
        SubClass sub1 = new SubClass("Joe Black", 21);
        SubClass sub2 = new SubClass("Jane Black", 20);
        SuperClass sup1 = new SuperClass("Adam");
        list.add(sub1);
        list.add(sub2);
        list.add(sup1);
    }

    public void print() {
        for (SuperClass e : list) {
            System.out.println(e); // uses print methods of SuperClass or SubClass
        }
    }

    public static void main(String args[]) {
        MyList theList = new MyList();
        theList.userInput();
        theList.print();
    }
}

预期产出：

$ javac SubClass.java SuperClass.java MyList.java
$ java -cp . MyList                              
Name: Joe Black
Age: 21
Name: Jane Black
Age: 20
Name: Adam
$ 

此外，我还有多个子类。

我现在遇到的问题是，它只打印实际输出的名称：

$ javac SubClass.java SuperClass.java MyList.java
$ java -cp . MyList                              
Name: Joe Black
Name: Jane Black
Name: Adam

Answer 1

为了方便起见，我将所有类放入一个文件中：

MyList.java

import java.util.ArrayList;
public class MyList {
    protected ArrayList<SuperClass> list = new ArrayList<>(); // display with Name and Age
    public void userInput() {
        // user inputting
        SubClass sub1 = new SubClass("Joe Black", 21);
        SubClass sub2 = new SubClass("Jane Black", 20);
        SuperClass sup1 = new SuperClass("Adam");
        list.add(sub1);
        list.add(sub2);
        list.add(sup1);
    }
    public void print() {
        for (SuperClass e : list) {
            if (e.getClass() == SuperClass.class)
            {
                // SuperClass has no attribute age. Therefore, create temporary SubClass instance
                SubClass s = new SubClass(e.name, 0);
                e = s;
            }
            System.out.println(e); // uses .toString of corresponding class 
        }
    }    
    public static void main(String args[]) {
        MyList theList = new MyList();
        theList.userInput();
        theList.print();
    }
}

class SuperClass {
        // field
    String name;
    // methods 
    public SuperClass(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Name: " + name;
    }
}

class SubClass extends SuperClass {
        // field 
    private int age;
    // methods
    public SubClass(String name, int age) {
        super(name);
        this.age = age;
    }

    @Override
    public String toString() {
        return "Name: " + name +
               "  Age: " + age;
    }
}

SuperClass没有属性age。因此，print方法使用精确的类检查，并创建一个临时SubClass实例，以便使用其toString方法。

这将产生以下输出。

$ javac MyList.java
$ java MyList      
Name: Joe Black  Age: 21
Name: Jane Black  Age: 20
Name: Adam  Age: 0
$ 

请注意，如果在SubClass中生成一个新的MyList.print实例，也可以通过其他方式修改输出，例如打印"Name: Adam：-“。MyList类可以随心所欲地处理SuperClass实例。