activity_main.xml
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    xmlns:android="http://schemas.android.com/apk/res/android">
          android:id="@+id/txtView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />
            android:id="@+id/myTxtB1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="请输入一个数"
        android:gravity="right"
        />
 

            android:id="@+id/myBtnAdd"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="+" />

            android:id="@+id/myBtnEql"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="=" />

MainActivity.java

package com.mycalc.org;

import android.os.Bundle;
import android.R.string;
import android.app.Activity;
import android.view.Gravity;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends Activity {
 private TextView txtV1=null;
 private EditText txtB1=null;
 private Button btnAdd=null;
 private Button btnEql=null;
 int op1=0;
 int op2=0;
 int result=0;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  txtV1 =(TextView)findViewById(R.id.txtView1);
  txtB1=(EditText)findViewById(R.id.myTxtB1);
  btnAdd=(Button)findViewById(R.id.myBtnAdd);
  btnEql=(Button)findViewById(R.id.myBtnEql);
  btnAdd.setOnClickListener(new MyClickListener());
  btnEql.setOnClickListener(new MyClickListener());
 }

 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  // Inflate the menu; this adds items to the action bar if it is present.
  getMenuInflater().inflate(R.menu.main, menu);
  return true;
 }
    class MyClickListener implements OnClickListener{

  @Override
  public void onClick(View v) {
   // TODO Auto-generated method stub
   if(v.getId()==btnAdd.getId()){
    //txtV1.setText("你点了加号！");
    op1=Integer.parseInt(txtB1.getText().toString());
    txtB1.setText("");
    
   }
   if(v.getId()==btnEql.getId())
   {
    op2=Integer.parseInt(txtB1.getText().toString());
    result=op1+op2;
    txtB1.setText(Integer.toString(result));
    txtB1.setSelection(txtB1.length());
   }
  }
     
    }

}