java Android Studio 应用不幸已停止

Android Studio App Unfortunately has stopped

提问by eustass

I'm new to Android and I was practicing, when i finished this practice app i ran it but it says "unfortunately has stopped" where is the problem?My app looks like a simple log in screen and i don't think there's a problem with the XML file.

我是 Android 新手，正在练习，当我完成这个练习应用程序时，我运行了它，但它说“不幸的是已停止”问题出在哪里？我的应用程序看起来像一个简单的登录屏幕，我认为没有XML 文件的问题。

package com.mycompany.testbutton;

import android.media.Image;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;



public class MainActivity extends ActionBarActivity {
Button buttonLog = (Button)findViewById(R.id.button1);
EditText userName = (EditText)findViewById(R.id.usernameText);
EditText passWord = (EditText)findViewById(R.id.passwordText);

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


    buttonLog.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String username = String.valueOf(userName.getText());
            String password = String.valueOf(passWord.getText());
            String allTogetherToast = "Your Email is " + username + " and your Password is " + password;
            Toast.makeText(MainActivity.this, allTogetherToast, Toast.LENGTH_LONG).show();


        }
    });
}


@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 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);
}

}

}

回答by Karthikeyan

Initialize these

初始化这些

Button buttonLog = (Button)findViewById(R.id.button1);
EditText userName = (EditText)findViewById(R.id.usernameText);
EditText passWord = (EditText)findViewById(R.id.passwordText);

after your

在你之后

 setContentView(R.layout.activity_main)

so your code will be like this

所以你的代码会是这样的

package com.mycompany.testbutton;

import android.media.Image;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;



public class MainActivity extends ActionBarActivity {
Button buttonLog; 
EditText userName; 
EditText passWord;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
buttonLog = (Button)findViewById(R.id.button1);
userName = (EditText)findViewById(R.id.usernameText);
passWord = (EditText)findViewById(R.id.passwordText);


buttonLog.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        String username = String.valueOf(userName.getText());
        String password = String.valueOf(passWord.getText());
        String allTogetherToast = "Your Email is " + username + " and your Password is " + password;
        Toast.makeText(MainActivity.this, allTogetherToast, Toast.LENGTH_LONG).show();


    }
});
}


@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 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;
}

回答by M D

Move

移动

Button buttonLog = (Button)findViewById(R.id.button1);
EditText userName = (EditText)findViewById(R.id.usernameText);
EditText passWord = (EditText)findViewById(R.id.passwordText);

after setContentView(....)inside onCreate(...)

在onCreate(...)setContentView(....)内部之后

Corrected:

更正：

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

Button buttonLog = (Button)findViewById(R.id.button1);
EditText userName = (EditText)findViewById(R.id.usernameText);
EditText passWord = (EditText)findViewById(R.id.passwordText);

Viewsavailable only after setContentView(....)

Views仅在之后可用 setContentView(....)