java - 尝试识别按下的按钮时出错

Question

作为 Android 新手，我尝试通过在按钮上输出 "Hello" 来识别按下的是哪个按钮，但在执行代码时遇到错误。

MainActivity.java

public class MainActivity extends AppCompatActivity implements View.OnClickListener  {

    Button button1 = findViewById(R.id.button1);
    Button button2 = findViewById(R.id.button2);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button1.setOnClickListener(this);
        button2.setOnClickListener(this);

    }


    @Override
    public void onClick(View v) {

        switch(v.getId())
        {
            case R.id.button1 :
                button1.setText("Hello");
                break;
            case R.id.button2 :
                button2.setText("Hello1");
                break;
            default:
                throw new RuntimeException("Unknown button ID");



        }

    }

}

上面的代码返回以下错误:

java.lang.RuntimeException: Unable to instantiate activity ...

Answer 1

问题是你的类属性:

Button button1 = findViewById(R.id.button1);
Button button2 = findViewById(R.id.button2);

在 onCreate 方法中调用 setContentView 之前，不能调用 findViewById。更改您的代码:

Button button1;
Button button2;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    button1 = findViewById(R.id.button1);
    button2 = findViewById(R.id.button2);

    button1.setOnClickListener(this);
    button2.setOnClickListener(this);

}