Java扩展示例

Java extends example

提问by Martin Kapfhammer

i have a java beginner question: Parent.print() prints "hallo" in the console, but also Child.print() prints "hallo". I thought it has to print "child". How can i solve this?

我有一个 Java 初学者问题：Parent.print() 在控制台中打印“hallo”，但 Child.print() 也会打印“hallo”。我认为它必须打印“孩子”。我该如何解决这个问题？

public class Parent {

    private String output = "hallo";

    public void print() {
        System.out.println(output);
    }

}

public class Child extends Parent {

   private String output = "child";

}

采纳答案by Jon Skeet

Currently you've got two separate variables, and the code in Parentonly knows about Parent.output. You need to set the value of Parent.outputto "child". For example:

目前你有两个单独的变量，并且代码Parent只知道Parent.output. 您需要将 的值设置Parent.output为“child”。例如：

public class Parent {

  private String output = "hallo";

  protected void setOutput(String output) {
    this.output = output;
  }

  public void print() {
    System.out.println(output );
  }
}

public class Child extends Parent {
  public Child() {
    setOutput("child");
  }
}

An alternative approach would be to give the Parent class a constructor which took the desired output:

另一种方法是为 Parent 类提供一个接受所需输出的构造函数：

public class Parent {
  private String output;

  public Parent(String output) {
    this.output = output;
  }

  public Parent() {
    this("hallo");
  }

  public void print() {
    System.out.println(output );
  }
}

public class Child extends Parent {
  public Child() {
    super("child");
  }
}

It really depends on what you want to do.

这真的取决于你想做什么。

回答by Jeremy

Childdoesn't have access to Parent's outputinstance variable because it is private. What you need to do is make it protectedand in the constructor of Childset outputto "child".

Child无法访问Parent的output实例变量，因为它是private. 您需要做的是protected在Childset outputto的构造函数中创建它"child"。

In other words, the two outputvariables are different.

换句话说，这两个output变量是不同的。

You could also do this if you change output to be protectedin Parent:

如果您将输出更改为protectedin ，您也可以这样做Parent：

public void print(){
    output = "child"
    super.print();
}

回答by Mike Miller

The reason why child is not printing "child" is that in inheritance in java, only methods are inherited, not fields. The variable outputis not overridden by the child.

child 不打印“child”的原因是在java中的继承中，只继承了方法，而不是字段。该变量output不会被孩子覆盖。

You could do it like this:

你可以这样做：

public class Parent {
    private String parentOutput = "hallo";

    String getOutput() {
        return output;
    }

    public void print() {
        System.out.println(getOutput());
    }
}

public class Child extends Parent {
    private String childOutput = "child";

    String getOutput() {
        return output;
    }
}

Also, the String variables do not need to be different names, but I did so here for clarity.

此外，字符串变量不需要是不同的名称，但为了清楚起见，我在这里这样做了。

Another, more readable way would be to do this:

另一种更具可读性的方法是这样做：

public class Parent {
    protected String output;

    public Parent() {
        output = "hallo";
    }

    public void print() {
        System.out.println(output);
    }
}

public class Child extends Parent {
    public Child() {
        output = "child";
    }
}

In this example the variable is protected, meaning it can be read from both the parent and child. The constructor of the classes sets the variable to the desired value. This way you only implement the print function once, and do not need a duplicate overridden method.

在这个例子中，变量是protected，这意味着它可以从父级和子级读取。类的构造函数将变量设置为所需的值。这样您只需实现一次打印功能，并且不需要重复的重写方法。

回答by KostasA

When I was trying to figure out the extend keyword, I was using two classes. I'll hope that also will help you to understand the basic idea.

当我试图找出 extend 关键字时，我使用了两个类。我希望这也能帮助你理解基本思想。

Parent.java

父程序

public class Parent {
    private int a1;
    private int b1;


    public Parent(int a, int b){
        this.a1 = a;
        this.b1 = b;
    }

    public void print() {
        System.out.println("a1= " + this.a1 + " b1= " + this.b1);
    }

}

Child.java

子程序

public class Child extends Parent {
        public Child(int c1, int d1){
        super(c1,d1);
    }

    public static void main(String[] args) {
        Parent pa = new Parent(1,2);
        pa.print();
        Child ch = new Child(5,6);
        ch.print();
    }

}