java - 在代码中创建布局并获取 nullPointerException

Question

我用纯代码创建我的布局。它奏效了，我决定创建另一个类来重用我的布局。

我添加了一个新类 (Designer.java) 以在其他 Activity 中重复使用我的页脚和页眉。

我将上下文作为参数传递给其他 Activity 。

• rel2MainActivity 有 3 个 child
• relMainActivity 为空

我不明白这是怎么回事。我一定是错过了什么

MainActivity.java

RelativeLayout relMainActivity ;

@Override
protected void onCreate(Bundle savedInstanceState) {

    // ...
    Designer design = new Designer();
    design.createDesign(relMainActivity, this);
    // relMainActivity = null in debug

    setContentView(relMainActivity);
    // java.lang.NullPointerException

}

Designer.java

RelativeLayout relHeader, relFooter, relContent;

public void createDesign(RelativeLayout rel2MainActivity, Context context) {

  rel2MainActivity = new RelativeLayout(context);
  // ...
  rel2MainActivity.addView(relHeader, 0);
  rel2MainActivity.addView(relContent, 1);
  rel2MainActivity.addView(relFooter, 2);
  // rel2MainActivity has 3 children in debug 

}

Answer 1

您在 createDesign() 中添加了 3 个 View ，但您没有将该 View 传递回被调用方。

因此，它将 null 赋给 relMainActivity

解决方案:

在这里当你打电话时:

    RelativeLayout relHeader, relFooter, relContent;

public RelativeLayout createDesign(RelativeLayout rel2MainActivity, Context context) {

  rel2MainActivity = new RelativeLayout(context);
  // ...
  rel2MainActivity.addView(relHeader, 0);
  rel2MainActivity.addView(relContent, 1);
  rel2MainActivity.addView(relFooter, 2);
  // rel2MainActivity has 3 children in debug 
   // rel2MainActivity view is local to this scope
   // you should return it
   return rel2MainActivity;


}

并在 :

RelativeLayout relMainActivity ;

@Override
protected void onCreate(Bundle savedInstanceState) {

    // ...
    Designer design = new Designer();
    relMainActivity = design.createDesign(relMainActivity, this);
    // relMainActivity = null in debug

    setContentView(relMainActivity);

}