java - 致命异常 : main and Null pointer exception arise while running a simple android program

Question

我试图在 Android 上做一些简单的事情。我创建了一个登录页面并继续工作。当我尝试创建弹出窗口时，抛出了致命异常。帮帮忙

NewProjectActivity.java

public class NewProjectActivity extends Activity {

    PopupWindow popUp;
    LinearLayout layout;
    TextView tv;
    LayoutParams params;
    LinearLayout mainLayout;
    boolean click = true;

    @Override
    protected void onCreate(Bundle savedInstanceState)  {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_new_project);
        TextView alertdetail = (TextView) findViewById(R.id.link_to_register);
        alertdetail.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if (click) {
                    popUp.showAtLocation(mainLayout, Gravity.BOTTOM, 10, 10);
                    popUp.update(50, 50, 300, 80);
                    click = false;
                } else {
                    popUp.dismiss();
                    click = true;
                }
            }
        });


        params = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
        layout.setOrientation(LinearLayout.VERTICAL);
        tv.setText("Hi this is a sample text for popup window");
        layout.addView(tv, params);
        popUp.setContentView(layout);
        popUp.showAtLocation(layout, Gravity.BOTTOM, 10, 10);
        mainLayout.addView(alertdetail, params);
        setContentView(mainLayout);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.new_project, menu);
        return true;
    }
}

Answer 1

TextView tv 未初始化，layout 和 popUp 和 mainLayout。

此外，您还为同一 Activity 设置了两次 setContentView，这并没有错，但设计很糟糕