android studio在简单请求时崩溃

Question

I am new to android studio and I have tried to write a simple program where the text "hello world" appears on the screen when a button is clicked, however, every time the button is clicked the program crashes.

activity_main.xml :

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.numsm_000.helloworld.MainActivity">

    <TextView
        android:id="@+id/text"
        android:layout_width="100dp"
        android:layout_height="100dp"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Click Me!"
        android:onClick="showMessage"
        android:id="@+id/button"
        android:layout_marginLeft="200dp"/>

</RelativeLayout>

MainActivity.java :

   package com.example.numsm_000.helloworld;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
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 showMessage(){
        TextView textView = (TextView) findViewById(R.id.text);
        textView.setText(R.string.app_name);
    }

}

Answer 1

Pass View as a parameter to showMessage() because onClick will look for a method which has View parameter

so it become

public void showMessage(View v){
    TextView textView = (TextView) findViewById(R.id.text);
    textView.setText(R.string.app_name);
}

Hope it helps :)

Answer 2

use following code 


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

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
 TextView textView;
Button btnClickMe;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = (TextView) findViewById(R.id.text);
      btnClickMe=(Button)findViewById(R.id.button);
       btnClickMe.setOnClickListener(this);

    }


 @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.button:
        textView.setText(R.string.app_name);
              break;

}

Answer 3

Try the following...

TextView textView = (TextView) findViewById(R.id.text);

Cut this line of code from showMessage() and put it in onCreate()... or you could just specify View view in ur method ..