以编程方式向布局添加按钮

Question

我在向XML创建的布局中添加按钮时遇到了问题.这就是我想要实现的目标:

//some class
else {
        startActivity(new Intent(StatisticsScreen.this, ScreenTemperature.class));
}
////

//ScreenTemperatureClass
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    //this is where I call another class that
    //displays a nice graph
    setContentView(new GraphTemperature(getApplicationContext()));

}

我想在Button这个新屏幕上添加一个,以便它显示在图表下方.我已经尝试创建一个LinearLayout视图,然后创建一个Button并将其添加到此视图但我只是得到了NullPointerException...

任何帮助,将不胜感激.谢谢

编辑#1

这是我尝试使用的创建了一个NullPointerException'强制关闭':

Button buybutton;
LinearLayout layout;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(new GraphTemperature(getApplicationContext()));

    layout = (LinearLayout) findViewById(R.id.statsviewlayout);
    Button buyButton = new Button(this);
    buyButton.setText(R.string.button_back);
    buyButton.setLayoutParams(new LayoutParams(
        ViewGroup.LayoutParams.WRAP_CONTENT,
            ViewGroup.LayoutParams.WRAP_CONTENT));
    layout.addView(buyButton);

}

这是logcat错误:

ERROR/AndroidRuntime(293): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.weatherapp/com.weatherapp.ScreenTemperature}: java.lang.NullPointerException
ERROR/AndroidRuntime(293):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
ERROR/AndroidRuntime(293):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
ERROR/AndroidRuntime(293):     at android.app.ActivityThread.access$2300(ActivityThread.java:125)
ERROR/AndroidRuntime(293):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)

在logcat中这个错误显然更多的线路,不确定你是否想要它？

编辑#2

所以我尝试了bhups方法:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    GraphTemperature GT = new GraphTemperature(getApplicationContext());             
    layout = (LinearLayout) findViewById(R.id.statsviewlayout);
    Button buyButton = new Button(this);
    buyButton.setText(R.string.button_back);
    buyButton.setLayoutParams(new LayoutParams(
        ViewGroup.LayoutParams.WRAP_CONTENT,
            ViewGroup.LayoutParams.WRAP_CONTENT));
    layout.addView(GT); // line 27
    layout.addView(buyButton);       
    setContentView(layout);           
}

此方法产生与上面相同的logcat错误NullPointerException,表明它与行号有关.27这是layout.addView代码行.有任何想法吗？再次感谢

Answer 1

如果您刚刚在onCreate()内部开头包含了一个布局文件,setContentView并希望以编程方式添加新元素,请尝试以下方法:

ViewGroup linearLayout = (ViewGroup) findViewById(R.id.linearLayoutID);

那么你可以创建一个新Button的例子,然后添加它:

Button bt = new Button(this);
bt.setText("A Button");
bt.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT, 
                                    LayoutParams.WRAP_CONTENT));
linerLayout.addView(bt);

Answer 2

这一行:

layout = (LinearLayout) findViewById(R.id.statsviewlayout);

在您当前的"contentview"中查找"statsviewlayout"ID.现在你在这里设置:

setContentView(new GraphTemperature(getApplicationContext()));

我猜测新的"graphTemperature"没有设置任何ID.

认为你可以用findViewById找到任何视图是一个常见的错误.您只能找到XML中的视图(或由代码指定并给出ID).

将抛出nullpointer,因为找不到您要查找的布局,所以

layout.addView(buyButton);

抛出那个例外.

另外:现在,如果您想从XML获取该视图,您应该使用inflater:

layout = (LinearLayout) View.inflate(this, R.layout.yourXMLYouWantToLoad, null);

假设您在名为"yourXMLYouWantToLoad.xml"的文件中包含linearlayout