java “MainActivity.this 不是封闭类”错误实际上是什么？

What's that "MainActivity.this is not enclosing class" error actually?

提问by Nomi Gd

I'm having problem in executing the following code.

我在执行以下代码时遇到问题。

Actually I wanted to use an image as a link to another page or activity. How it's done

实际上，我想将图像用作指向另一个页面或活动的链接。它是如何完成的

and

和

what's that "MainActivity.this is not enclosing class" problem actually? Here is a code

“MainActivity.this 不是封闭类”问题实际上是什么？这是一个代码

I have 2 classes, 1st is MainActivity

我有 2 个类，第一个是 MainActivity

and

和

2nd is MainActivity2

第二个是 MainActivity2

both are in Single MainAcitvity.java file

两者都在 Single MainAcitvity.java 文件中

MainActivity:

主要活动：

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageButton;
import android.widget.Toast;
import android.content.Intent;
import android.util.Log;


public class MainActivity extends AppCompatActivity {
ImageButton androidImageButton;
        @Override
    protected void onCreate (Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });





        androidImageButton = (ImageButton) findViewById(R.id.imageButton_android);
        androidImageButton.setOnClickListener (new View.OnClickListener(){

            @Override
            public void onClick(View v) {
                Toast.makeText(MainActivity.this, "It works", Toast.LENGTH_LONG).show();
            }

            });



    }

    public boolean onCreateOptionsMenu (Menu menu){
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    public boolean onOptionsItemSelected (MenuItem item){
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

MainActivity2:

主要活动2：

class MainActivity2 extends MainActivity implements OnClickListener, View.OnClickListener {

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

    ImageButton Btn = (ImageButton) findViewById(R.id.imageButton_android);
    Btn.setOnClickListener(this);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public void onClick(View v) {
    Log.i("clicks","You Clicked B1");
    Intent i;
    i = new Intent( MainActivity.this, MainActivity2.class); *//The problem is here wih MainActivity.this//
    startActivity(i);
}
}

Can any one please resolve the error and help me out with the code.

任何人都可以解决错误并帮助我解决代码。

采纳答案by adelphus

An enclosing classis exactly what it sounds like - it's a class that encloses(not inherits) the class at the given statement. In order to reference the enclosing class instance, the thiskeyword must be prefixed with the class name - hence MainActivity.this.

一个封闭类正是这听起来像-这是一类封闭（不继承）在给定的语句类。为了引用封闭的类实例，this关键字必须以类名作为前缀 - 因此MainActivity.this.

class ABC {
    class XYZ extends Activity {
    } 
}

In the simple example above, ABCis the enclosingclass of XYZ.

在上面的简单的例子，ABC是封闭类的XYZ。

Your error is telling you that MainActivityis not an enclosing class at the statement location, and so the thisinstance of that class cannot be accessed.

您的错误告诉您这MainActivity不是语句位置的封闭类，因此this无法访问该类的实例。

Your MainActivity2class inheritsfrom MainActivity, but there is no enclosingclass at the Intent(...)statement. Since the Intent()constructor requires a Contextparameter and your MainActivity2thisinstance inherits from Context(Context -> Activity -> MainActivity -> MainActivity2), you can just use thisas the parameter:

您的MainActivity2类继承自MainActivity，但语句中没有 封闭类Intent(...)。由于Intent()构造函数需要一个Context参数，并且您的MainActivity2this实例继承自Context(Context -> Activity -> MainActivity -> MainActivity2)，因此您可以将其this用作参数：

So instead of:

所以而不是：

 i = new Intent( MainActivity.this, MainActivity2.class); 

use:

利用：

 i = new Intent(this, MainActivity2.class);