android studio，按钮对象无法识别，

Question

导入View和Button后。我为Button创建了一个对象，在本例中是“thomasButton”。

但错误表明字段'thomasButton'没有被使用，即使我在下一行调用它。

过了一会儿，我发现如果把它放在另一个范围内就可以识别该字段。像这样，但程序仍然不会运行（启动时崩溃）。你们知道setOnLongClickListener用于按钮的正确方法是什么？

package com.example.thoma.event;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

public void displayMessage(View v){
    TextView thomasText = (TextView)findViewById(R.id.txt_View);
    thomasText.setText(R.string.rsc_Text2);x
}

Button thomasButton = (Button)findViewById(R.id.btn_Change);
// weird but I can only use Button object within an inner scope
{
    thomasButton.setOnLongClickListener(new View.OnLongClickListener() {
        public boolean onLongClick(View v) {
            TextView thomasText = (TextView) findViewById(R.id.txt_View);
            thomasText.setText("Artificial");
            // it will return false if long click wasnt long enough
            // and normal click will be called
            return true;
        }
    });
}
}

崩溃：不幸的是，App已经停止了

Answer 1

按钮初始化在onCreate()检查下面的代码。 findViewById是昂贵的，所以如果你尽可能地发现这些id会更好

package com.example.thoma.event;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

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

     TextView thomasText = (TextView) findViewById(R.id.txt_View);

     thomasButton.setOnLongClickListener(new View.OnLongClickListener() {
        public boolean onLongClick(View v) {
            thomasText.setText("Artificial");

            // it will return false if long click wasnt long enough
            // and normal click will be called

            return true;
        }
    });

}

public void displayMessage(View v){
    TextView thomasText = (TextView)findViewById(R.id.txt_View);
    thomasText.setText(R.string.rsc_Text2);
}


}

Answer 2

package com.example.thoma.event;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
TextView thomasText;
Button thomasButton;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    thomasText = (TextView)findViewById(R.id.txt_View);
      thomasButton = (Button)findViewById(R.id.btn_Change);
      thomasButton.setOnLongClickListener(new View.OnLongClickListener() {
        public boolean onLongClick(View v) {
             thomasText.setText("Artificial");
            // it will return false if long click wasnt long enough
            // and normal click will be called
            return true;
        }
    });

}

public void displayMessage(View v){
    thomasText.setText(R.string.rsc_Text2);
}



}

Answer 3

试试这个

    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.TextView;

    public class MainActivity extends AppCompatActivity {

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

         Button thomasButton = (Button)findViewById(R.id.btn_Change);
         thomasButton.setOnLongClickListener(new View.OnLongClickListener() {
                @Override
                public boolean onLongClick(View v) {
                    TextView thomasText = (TextView) findViewById(R.id.txt_View);
                    thomasText.setText("Artificial");
                    return false;
                }
            });
    }


    }