以编程方式将按钮添加到布局 [英] Add button to a layout programmatically

问题描述

我无法向我用 XML 创建的布局添加按钮.这是我想要实现的目标:

//某个类别的 {开始活动(新意图(StatisticsScreen.this，ScreenTemperature.class))；}//////屏幕温度类@覆盖public void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);//这是我调用另一个类的地方//显示一个漂亮的图形setContentView(new GraphTemperature(getApplicationContext()));}

我想在这个新屏幕上添加一个 Button，这样它就会出现在图表下方.我尝试创建一个 LinearLayout 视图，然后创建一个 Button 并将其添加到此视图中，但我只得到 NullPointerExceptions..

任何帮助将不胜感激.谢谢

编辑#1

这是我尝试使用的创建 NullPointerException 和强制关闭"的方法:

按钮购买按钮；线性布局布局；@覆盖public void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(new GraphTemperature(getApplicationContext()));layout = (LinearLayout) findViewById(R.id.statsviewlayout);按钮buyButton = new Button(this);buyButton.setText(R.string.button_back);buyButton.setLayoutParams(新的LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT，ViewGroup.LayoutParams.WRAP_CONTENT));layout.addView(buyButton);}

这里是 logcat 错误:

ERROR/AndroidRuntime(293): java.lang.RuntimeException: 无法启动活动 ComponentInfo{com.weatherapp/com.weatherapp.ScreenTemperature}: java.lang.NullPointerException错误/AndroidRuntime(293): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)错误/AndroidRuntime(293): 在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)错误/AndroidRuntime(293): 在 android.app.ActivityThread.access$2300(ActivityThread.java:125)错误/AndroidRuntime(293): 在 android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)

显然在 logcat 中有更多行与此错误有关，不知道您是否需要?

编辑#2

所以我尝试了 bhups 方法:

@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);GraphTemperature GT = new GraphTemperature(getApplicationContext());layout = (LinearLayout) findViewById(R.id.statsviewlayout);按钮buyButton = new Button(this);buyButton.setText(R.string.button_back);buyButton.setLayoutParams(新的LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT，ViewGroup.LayoutParams.WRAP_CONTENT));layout.addView(GT);//第 27 行layout.addView(buyButton);设置内容视图(布局)；}

此方法产生了与上面相同的 logcat 错误，NullPointerException，表明它与第 1 行有关.27 这是 layout.addView 代码行.有任何想法吗?再次感谢

解决方案

这一行:

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

在您当前的内容视图"中查找statsviewlayout"ID.现在你已经在这里设置了:

setContentView(new GraphTemperature(getApplicationContext()));

而且我猜新的graphTemperature"没有设置任何具有该 ID 的内容.

认为您可以使用 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(或由代码指定并给定id).

空指针会被抛出，因为找不到你要找的布局，所以

layout.addView(buyButton);

抛出那个异常.

补充:现在，如果您想从 XML 获取该视图，则应使用充气器:

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

假设您在名为yourXMLYouWantToLoad.xml"的文件中有您的线性布局

I'm having trouble adding a button to a layout that I've created in XML. Here's what I want to achieve:

//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()));

}

I want to add a Button to this new screen so that it'll appear below the graph. I've tried creating a LinearLayout view, then create a Button and add it to this view but I just get NullPointerExceptions..

Any help would be appreciated. Thanks

EDIT#1

Here's what I've tried using that created a NullPointerException and 'force close':

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);

}

And here's the logcat error:

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)

theres abviously more lines to do with this error in logcat, not sure if you want it?

EDIT#2

So i tried bhups method:

@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);           
}

This method produced the same logcat error as above, NullPointerException, indicating it was something to do with line no. 27 which is the layout.addView line of code. Any ideas? Thanks again

解决方案

This line:

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

Looks for the "statsviewlayout" id in your current 'contentview'. Now you've set that here:

setContentView(new GraphTemperature(getApplicationContext()));

And i'm guessing that new "graphTemperature" does not set anything with that id.

It's a common mistake to think you can just find any view with findViewById. You can only find a view that is in the XML (or appointed by code and given an id).

The nullpointer will be thrown because the layout you're looking for isn't found, so

layout.addView(buyButton);

Throws that exception.

addition: Now if you want to get that view from an XML, you should use an inflater:

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

assuming that you have your linearlayout in a file called "yourXMLYouWantToLoad.xml"

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