最最简单的多线程

<LinearLayout 
    android:layout_width="match_parent" 
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">
    
    <EditText 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/etNum"
        android:hint="input"
        android:selectAllOnFocus="true"
        />
    <Button 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="按下去"
        android:id="@+id/btn"/>
</LinearLayout>

package com.example.deemo;

import java.util.ArrayList;
import java.util.List;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity {
    
    static final String UPPER_NUM = "upper";
    EditText etNum;
    CalThread calThread;
    private Button button;
    
    //定义一个线程类
    class CalThread extends Thread{
        public Handler mHandler;
        public void run(){//执行方法
            Looper.prepare();
            
            mHandler = new Handler(){
                //定义消息处理方法
                @Override
                public void handleMessage(Message msg){
                    if(msg.what == 0x123){
                        int upper = msg.getData().getInt(UPPER_NUM);
                        List<Integer> nums = new ArrayList<Integer>();
                        
                        //计算方法
                        outer:
                            for (int i = 2; i <= upper; i++) {
                                for (int j = 2; j <= Math.sqrt(i); j++) {
                                    if(i != 2 && i%j ==0){
                                        continue outer;
                                    }
                                }
                                nums.add(i);
                            }
                        Toast.makeText(MainActivity.this, nums.toString(), Toast.LENGTH_LONG).show();;
                    }
                }
            };
            Looper.loop();
        }
    }
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        etNum = (EditText) findViewById(R.id.etNum);
        calThread = new CalThread();
        calThread.start();//启动新线程
        initClick();//点击事件
       
    }

    private void initClick() {
        button = (Button) findViewById(R.id.btn);
        button.setOnClickListener(new OnClickListener() {
            
            @Override
            public void onClick(View arg0) {
                //创建消息
                Message msg = new Message();
                msg.what = 0x123;
                Bundle bundle = new Bundle();
                bundle.putInt(UPPER_NUM, Integer.parseInt(etNum.getText().toString()));
                msg.setData(bundle);
                //向新线程中发送消息
                calThread.mHandler.sendMessage(msg);
            }
        });    
    }
}