覆盖 child class 中的 parent 方法。 Child 方法什么都不做

Question

我遇到了一些麻烦。我遵循了所有在线指南，展示了如何覆盖 child class 中的 parent 方法。我已经完成了我被告知要做的所有事情，但我的 child 函数什么也没做。

我的 MainActivity(Parent) class:

package com.example.flashcards;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    DatabseHelper DB = new DatabseHelper(this);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        changeText();
        changeText2();
    };
    public void changeText(){}
    public void changeText2(){}
        String [] columns = new String[] {
                DatabseHelper.FLASHCARD_QUESTION,
                DatabseHelper.FLASHCARD_ANSWER
        };

    @Override
    public void onClick(View v) {

    }
}

我的 child class (TextC)

package com.example.flashcards;

import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class TextC extends MainActivity {

    @Override
    public void changeText() {
        super.changeText();
        final String[] revertText = {"H2O", "What elements does water consist of?"};

        final TextView textChange = findViewById(R.id.flashcard1);
        Button change = findViewById(R.id.answer1);

        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int random = (int) (Math.random() * revertText.length);
                textChange.setText(revertText[random]);
            }
        });
    }

    public void changeText2() {
        super.changeText2();
        final String[] revertText = {"2,200° F", "How hot does lava get?"};

        final TextView textChange = findViewById(R.id.flashcard2);
        Button change = findViewById(R.id.answer2);

        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int random = (int) (Math.random() * revertText.length);
                textChange.setText(revertText[random]);
            }
        });
    }
}

我的 changeText() 函数什么都不做。我没有收到任何错误，所以我不知道我做错了什么。我是否需要为 child class 创建一个 onCreate 方法？但是我正在扩展具有它的 MainActivity。

关于为什么我的方法覆盖不起作用的任何想法？

Answer 1

有了继承和覆盖的概念，您需要在 child class 中覆盖 onCreate 函数。从那个覆盖的方法中，您可以调用 super.onCreate 或者您可以从 child class.

执行 this.chnageText 和 this.changeText2

否则，当您调用 onCreate 函数时，它只会从 super class 调用 changeText 和 changeText2。

在你的childclass

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        this.changeText();
        this.changeText2();
    };

Answer 2

修改parentclass

package com.example.flashcards;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    DatabseHelper DB = new DatabseHelper(this);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //changeText(); freeze or remove these callings
        //changeText2();
    };
    public void changeText(){}
    public void changeText2(){}

}

并向您的 child class

添加一些代码

package com.example.flashcards;

import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class TextC extends MainActivity {

 @Override
 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    changeText(); 
    changeText2();
};

    @Override
    public void changeText() {
        super.changeText();
        final String[] revertText = {"H2O", "What elements does water consist of?"};

        final TextView textChange = findViewById(R.id.flashcard1);
        Button change = findViewById(R.id.answer1);

        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int random = (int) (Math.random() * revertText.length);
                textChange.setText(revertText[random]);
            }
        });
    }

    public void changeText2() {
        super.changeText2();
        final String[] revertText = {"2,200° F", "How hot does lava get?"};

        final TextView textChange = findViewById(R.id.flashcard2);
        Button change = findViewById(R.id.answer2);

        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int random = (int) (Math.random() * revertText.length);
                textChange.setText(revertText[random]);
            }
        });
    }
}

在上面的方法中没有声明changeText()；和 changeText2();在 parent activity 中。为了可重用性，我们可以使用抽象classes和方法。

对您的 parent activity 进行一些更改，如下所示。

public abstract class MainActivity extends AppCompatActivity implements View.OnClickListener {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      changeText(); // they dont have default implimentation in parent so it will be invoked from child class where these methods implimented
      changeText2();
   };


public abstract void changeText(); //there is no default implimentation

public abstract void changeText2();

}

并且在 child activity 中，您必须实现这些方法。

public class TextC extends MainActivity {

 @Override
 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //changeText(); no need to call these methods bcz its already called in parent onCreate()
    //changeText2();
};

    @Override
    public void changeText() {
        super.changeText();
        final String[] revertText = {"H2O", "What elements does water consist of?"};

        final TextView textChange = findViewById(R.id.flashcard1);
        Button change = findViewById(R.id.answer1);

        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int random = (int) (Math.random() * revertText.length);
                textChange.setText(revertText[random]);
            }
        });
    }

    public void changeText2() {
        super.changeText2();
        final String[] revertText = {"2,200° F", "How hot does lava get?"};

        final TextView textChange = findViewById(R.id.flashcard2);
        Button change = findViewById(R.id.answer2);

        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int random = (int) (Math.random() * revertText.length);
                textChange.setText(revertText[random]);
            }
        });
    }
}

覆盖 child class 中的 parent 方法。 Child 方法什么都不做

Overriding parent method in child class. Child method not doing anything

java

android

overriding

super