为什么这段代码不能改变Android上的文字？

Question

This code to change text when the user presses a button doesn't work. Why?

package com.cookbook.simple_activity;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class activity extends Activity {
    private TextView txt = (TextView) findViewById(R.id.hello_text);
    Button startButton = (Button) findViewById(R.id.trigger);
   @Override
   public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_simple);


   startButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view) {
            txt.setText(R.string.pressthisbutton);
        }
    });
 }
}

Answer 1

Change it to

    public class activity extends Activity {
    private TextView txt;
    Button startButton;
   @Override
   public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_simple);
        txt = (TextView) findViewById(R.id.hello_text);
        startButton = (Button) findViewById(R.id.trigger);

You need to initialize your Views after inflating your layout with setContentView() . Since your Views exist in your layout , they will return null if you haven't inflated your layout first. You can declare them before your setContentView() but you can't initialize them until after.

Also, since you are trying to access txt inside your listener it must either be final or declared as a member variable as above.

This was a rather easy one to spot but they aren't always. When you post a question try to describe what isn't working and how. Here it would be a NPE I'm guessing when you try to set the listener on your Button so it crashes. When it does crash, please provide the logcat so it is easier for us to spot the problem.

Edit

After seeing your xml, you are using the same string value for your initial text as you are for the text when you click the Button so of course it doesn't change. I'm not sure what you're after here but the code is running as it should as it is written.

Answer 2

try this code ....

you need to declare the

 TextView txt = (TextView) findViewById(R.id.hello_text); 

 Button startButton = (Button) findViewById(R.id.trigger);

after the create method otherwise it doesnot recognize id android Activity lifecycle starts with the

onCreate() method

package com.cookbook.simple_activity;

    import android.app.Activity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.TextView;

    public class activity extends Activity {
       @Override
       public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_simple);

        TextView txt = (TextView) findViewById(R.id.hello_text);
        Button startButton = (Button) findViewById(R.id.trigger);

       startButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                txt.setText(R.string.pressthisbutton);
            }
        });
     }
    }

Answer 3

The problem is your layout is android_simple

setContentView(R.layout.activity_simple);

but you are setting contentView as activity_simple

EDIT: 1 Do this

Initiallization of you textView and button in onCreate because this setContentView(R.layout.activity_simple); says where to find the view but you set the view before telling the layout file

TextView txt = null;
Button startButton = null;

 @Override
   public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_simple);
    txt = (TextView) findViewById(R.id.hello_text);
    startButton = (Button) findViewById(R.id.trigger);

   startButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view) {
            txt.setText(R.string.pressthisbutton);
        }
    });
 }

Answer 4

尝试这个：

txt.setText(getResources().getString(R.string.pressthisbutton));