Android 在启动期间获取NullPointerException

我有九个按钮，每个按钮在编辑文本中输入一个数字，有点像计算器。我只是作为一个简单的项目来做这件事，但我一直收到这些NullPointerExeption。如果我只使用预先制作的onCreate（）方法运行它，它似乎可以工作，但一旦我开始在类声明下声明变量，它就会抛出一些异常。感谢您的帮助。谢谢

public class MainActivity extends Activity implements OnClickListener {

EditText display = (EditText)findViewById(R.id.numdisplay);
Button clearbtn = (Button)findViewById(R.id.clear);
Button ninenum = (Button)findViewById(R.id.nine);
Button eightnum = (Button)findViewById(R.id.eight);
Button sevennum = (Button)findViewById(R.id.seven);
Button sixnum = (Button)findViewById(R.id.six);
Button fivenum = (Button)findViewById(R.id.five);
Button fournum = (Button)findViewById(R.id.four);
Button threenum = (Button)findViewById(R.id.three);
Button twonum = (Button)findViewById(R.id.two);
Button onenum = (Button)findViewById(R.id.one);
Button enterbtn = (Button)findViewById(R.id.enter);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    onclicks(); //setting all the onclick listeners
}

public void onclicks() { //just setting onclick listeners, so it doesn't bloat up the oncreate method
    clearbtn.setOnClickListener(this);
    ninenum.setOnClickListener(this);
    eightnum.setOnClickListener(this);
    sevennum.setOnClickListener(this);
    sixnum.setOnClickListener(this);
    fivenum.setOnClickListener(this);
    fournum.setOnClickListener(this);
    threenum.setOnClickListener(this);
    twonum.setOnClickListener(this);
    onenum.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.main, menu);
    return true;
}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub
    switch(v.getId()){
    case R.id.clear: /** More switch cases will be put here*/
           display.setText("");
           break;
}
}
}

放置此代码

ditText display = (EditText)findViewById(R.id.numdisplay);
Button clearbtn = (Button)findViewById(R.id.clear);
Button ninenum = (Button)findViewById(R.id.nine);
Button eightnum = (Button)findViewById(R.id.eight);
Button sevennum = (Button)findViewById(R.id.seven);
Button sixnum = (Button)findViewById(R.id.six);
Button fivenum = (Button)findViewById(R.id.five);
Button fournum = (Button)findViewById(R.id.four);
Button threenum = (Button)findViewById(R.id.three);
Button twonum = (Button)findViewById(R.id.two);
Button onenum = (Button)findViewById(R.id.one);
Button enterbtn = (Button)findViewById(R.id.enter);

在

setContentView

因为

只有在

onCreate（）

中

setContentView

才能获取视图引用，因此当您试图在

onCreate（）之外获取引用时，您将获得
null
引用，访问
null
将获得
NullPointerException

这样申报

Button clearbtn ,ninenum ,eightnum ,sevennum ,sixnum ,fivenum ,fivenum ,fournum ,threenum ,twonum ,onenum ,enterbtn ;
EditText display;

在onCreate（）中

只有在设置ContentView布局后，Findviewbyid才会返回。你所做的是在课堂上尝试重新学习

尝试添加

EditText display = (EditText)findViewById(R.id.numdisplay);
Button clearbtn = (Button)findViewById(R.id.clear);
Button ninenum = (Button)findViewById(R.id.nine);
Button eightnum = (Button)findViewById(R.id.eight);
Button sevennum = (Button)findViewById(R.id.seven);
Button sixnum = (Button)findViewById(R.id.six);
Button fivenum = (Button)findViewById(R.id.five);
Button fournum = (Button)findViewById(R.id.four);
Button threenum = (Button)findViewById(R.id.three);
Button twonum = (Button)findViewById(R.id.two);
Button onenum = (Button)findViewById(R.id.one);
Button enterbtn = (Button)findViewById(R.id.enter);

在
setContentView
语句之后
setContentView（）
用于将活动内容设置为显式视图，因此在引用视图之前，不能使用其中的控件

试试下面的代码

public class MainActivity extends Activity implements OnClickListener {
EditText display;
Button clearbtn,ninenum,eightnum,sevennum,sixnum,fivenum,fournum,threenum,twonum,onenum,enterbtn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    display = (EditText)findViewById(R.id.numdisplay);
    onclicks(); //setting all the onclick listeners
}

public void onclicks() { //just setting onclick listeners, so it doesn't bloat up the oncreate method
    clearbtn.setOnClickListener(this);
    ninenum.setOnClickListener(this);
    eightnum.setOnClickListener(this);
    sevennum.setOnClickListener(this);
    sixnum.setOnClickListener(this);
    fivenum.setOnClickListener(this);
    fournum.setOnClickListener(this);
    threenum.setOnClickListener(this);
    twonum.setOnClickListener(this);
    onenum.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.main, menu);
    return true;
}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub
    switch(v.getId()){
    case R.id.clear: /** More switch cases will be put here*/
           display.setText("");
           break;
}
}
}

希望这有帮助。
它仍在抛出异常。这就是最初发生的事情，我不知道为什么。我确保onclicklisteners和edittext放在SetContentView部分之后。@ThatGuyThere:尝试用上面的代码替换完整的代码…如果问题仍然存在，请将日志放在这里
public class MainActivity extends Activity implements OnClickListener {
EditText display;
Button clearbtn,ninenum,eightnum,sevennum,sixnum,fivenum,fournum,threenum,twonum,onenum,enterbtn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    display = (EditText)findViewById(R.id.numdisplay);
    onclicks(); //setting all the onclick listeners
}

public void onclicks() { //just setting onclick listeners, so it doesn't bloat up the oncreate method
    clearbtn.setOnClickListener(this);
    ninenum.setOnClickListener(this);
    eightnum.setOnClickListener(this);
    sevennum.setOnClickListener(this);
    sixnum.setOnClickListener(this);
    fivenum.setOnClickListener(this);
    fournum.setOnClickListener(this);
    threenum.setOnClickListener(this);
    twonum.setOnClickListener(this);
    onenum.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.main, menu);
    return true;
}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub
    switch(v.getId()){
    case R.id.clear: /** More switch cases will be put here*/
           display.setText("");
           break;
}
}
}