在启动期间获取NullPointerException

Question

我有九个按钮，每个按钮都以数字形式打到EditText，有点像计算器。我只是将其作为一个简单的项目来完成，但我一直在接收这些NullPointerExeptions。如果我只用预先制作的onCreate（）方法运行它，它似乎工作，但一旦我开始在类声明下声明变量，它会抛出一些异常。任何帮助表示赞赏。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; 
} 
} 
} 

Answer 1

将此代码

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); 

的onCreate()内后setContentView

因为

，你不会得到你的意见，直到您setContentView引用onCreate()，所以当你试图要获得onCreate()以外的参考，您将获得null参考，并访问null会给你NullPointerException

声明这样

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

而在的onCreate（）

setContentView(R.layout.activity_main); 

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

Answer 2

Findviewbyid将返回你的setContentView该布局之后。你正在做的是试图在课堂负荷上进行回顾。

尝试增加

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); 

后setContentView声明

Answer 3

setContentview()用来设置活动内容，以明确的看法，所以引用到一个视图之前，你不能在使用的控件。

尝试下面的代码

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; 
} 
} 
} 

希望这会有所帮助。