这是我的代码：我的计算器项目的清单文件： 当我尝试在运行android 4.4.2 kitkat版本的手机上启动应用程序时，它表示计算器已停止强行关闭。
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.jeet.calculator"
android:versionCode="1"
android:versionName="1.0">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
这是我的xml代码：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<ScrollView
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="0dp"
android:layout_weight= "30">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:id="@+id/textview1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="15dp"
android:layout_gravity="right"
android:gravity="right"
android:padding="10dp"
android:text=""/>
<TextView
android:id="@+id/textview2"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="10dp"
android:layout_gravity="right"
android:gravity="right"
android:padding="10dp"
android:text=""
android:layout_marginBottom="10dp" />
<Button
android:id="@+id/clear"
style="?android:attr/buttonStyleSmall"
android:layout_width="260dp"
android:layout_height="wrap_content"
android:gravity="center"
android:text="Clear"
android:layout_marginLeft="60dp" />
<LinearLayout
android:layout_width="fill_parent"
android:gravity="center"
android:layout_marginTop="10dp"
android:orientation="horizontal"
android:layout_height="wrap_content">
<Button
android:id="@+id/one"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="1" />
<Button
android:id="@+id/two"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="2"/>
<Button
android:id="@+id/three"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="3"/>
<Button
android:id="@+id/add"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="+"/>
</LinearLayout>
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="horizontal">
<Button
android:id="@+id/four"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="4"/>
<Button
android:id="@+id/five"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="5"/>
<Button
android:id="@+id/six"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="6"/>
<Button
android:id="@+id/minus"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="-"/>
</LinearLayout>
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="horizontal">
<Button
android:id="@+id/seven"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="7"/>
<Button
android:id="@+id/eight"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="8"/>
<Button
android:id="@+id/nine"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="9"/>
<Button
android:id="@+id/divide"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="/"/>
</LinearLayout>
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="horizontal">
<Button
android:id="@+id/ce"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="CE"/>
<Button
android:id="@+id/zero"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="0"/>
<Button
android:id="@+id/equal"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="="/>
<Button
android:id="@+id/mul"
android:layout_width="65dp"
android:layout_height="55dp"
android:text="*"/>
</LinearLayout>
</LinearLayout>
</ScrollView>
</LinearLayout>
以下是Java中的MainActivity代码：
package com.example.jeet.calculator;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
Button b1, b2, b3, b4, b5, b6, b7, b8, b9, bzero, bAdd, bSub, bMul, bDiv, bClear, bEqual,bCe;
TextView txt1, txt2;
String s = "", s1 = "" , s2 = "" , resultString = "";
int i =0 , i1 = 0, c = -1;
int result = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.real);
b1 = (Button)findViewById(R.id.one);
b2 = (Button) findViewById(R.id.two);
b3 = (Button)findViewById(R.id.three);
b4 = (Button)findViewById(R.id.four);
b5 = (Button)findViewById(R.id.five);
b6 = (Button)findViewById(R.id.six);
b7 = (Button)findViewById(R.id.seven);
b8 = (Button)findViewById(R.id.eight);
b9 = (Button)findViewById(R.id.nine);
bzero = (Button)findViewById(R.id.zero);
bAdd = (Button)findViewById(R.id.add);
bSub = (Button)findViewById(R.id.minus);
bMul = (Button)findViewById(R.id.mul);
bDiv = (Button)findViewById(R.id.divide);
bClear = (Button)findViewById(R.id.clear);
bCe = (Button)findViewById(R.id.ce);
txt1 = (TextView)findViewById(R.id.textview1);
txt2 = (TextView)findViewById(R.id.textview2);
b1.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
s = (String) txt1.getText();
if (s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) {
txt1.setText("");
s = "";
}
txt1.setText(s + "1");
s = "";
}
});
b2.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
s = (String) txt1.getText();
if (s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) {
txt1.setText("");
s = "";
}
txt1.setText(s + "2");
s = "";
}
});
b3.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) {
txt1.setText("");
s = "";
}
txt1.setText(s + "3");
s = "";
}
});
b4.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "4");
s = "";
}
});
b5.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "5");
s = "";
}
});
b6.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "6");
s = "";
}
});
b7.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "7");
s = "";
}
});
b8.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "8");
s = "";
}
});
b9.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "9");
s = "";
}
});
bzero.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
txt1.setText("");
s = "";
}
txt1.setText(s + "0");
s = "";
}
});
bCe.setOnClickListener(new View.OnClickListener(){
public void onClick(View v) {
txt1.setText("");
txt2.setText("");
i = 0;
i1 = 0;
s1 = "";
s2 = "";
resultString = "";
c = -1;
result = 0;
}
});
bClear.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
s = (String)txt1.getText();
if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/") || s.equals("")){
i = 0;
}
else{
i = Integer.parseInt(s);
i = i/10;
}
if(i == 0){
txt1.setText("");
}
else{
txt1.setText(i + "");
}
s = null;
}
});
bAdd.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
String tmp = (String)txt1.getText();
if(tmp.isEmpty()){
s1 = "0";
}
else if(!tmp.equals("+") && !tmp.equals("-") && !tmp.equals("*") && !tmp.equals("/")){
s1 = tmp;
}
c = 0;
resultString = "";
txt1.setText("+");
txt2.setText(s1 + " + ");
}
});
bSub.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
String tmp = (String)txt1.getText();
if(tmp.isEmpty()){
s1 = "0";
}
else if(!tmp.equals("+") && !tmp.equals("-") && !tmp.equals("*") && !tmp.equals("/")){
s1 = tmp;
}
c= 1;
resultString = "";
txt1.setText("-");
txt2.setText(s1 + " - ");
}
});
bMul.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
String tmp = (String)txt1.getText();
if(tmp.isEmpty()){
s1 = "0";
}
else if(!tmp.equals("+") && !tmp.equals("-") && !tmp.equals("*") && !tmp.equals("/")){
s1 = tmp;
}
c= 3;
resultString = "";
txt1.setText("*");
txt2.setText(s1 + " * ");
}
});
bDiv.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
String tmp = (String)txt1.getText();
if(tmp.isEmpty()){
s1 = "0";
}
else if(!tmp.equals("+") && !tmp.equals("-") && !tmp.equals("*") && !tmp.equals("/")){
s1 = tmp;
}
c= 2;
resultString = "";
txt1.setText("/");
txt2.setText(s1 + " / ");
}
});
bEqual.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String operator = "";
if (s1.equalsIgnoreCase("+") || s1.equalsIgnoreCase("-") || s1.equalsIgnoreCase("/") || s1.equalsIgnoreCase("*")) {
i = 0;
} else if (s1 == null || s1.isEmpty()) {
i = 0;
} else {
i = Integer.parseInt(s1);
}
if(resultString.isEmpty()) {
s2 = (String) txt1.getText();
if (s2.equalsIgnoreCase("+") || s2.equalsIgnoreCase("-") || s2.equalsIgnoreCase("/") || s2.equalsIgnoreCase("*")) {
i1 = 0;
} else if (s2 == null || s2.isEmpty()) {
i1 = 0;
} else {
i1 = Integer.parseInt(s2);
}
} else {
i = result;
}
if (c == 0) {
operator = "+";
result = i + i1;
} else if (c == 1) {
operator = "-";
result = i - i1;
} else if (c == 2) {
operator = "/";
if (i1 == 0) {
Toast.makeText(getApplicationContext(),
"Invalid Input", Toast.LENGTH_LONG).show();
result = 0;
} else {
result = i / i1;
}
} else if (c == 3) {
operator = "*";
result = i * i1;
} else {
operator = "";
result = 0;
}
//History Storage
if(!operator.isEmpty()) {
txt2.setText(i + " " + operator + " " + i1);
} else {
txt2.setText("");
}
resultString = String.valueOf(result);
txt1.setText(resultString);
}
});
}
}
Logcat错误如下：
**02-09 23:39:17.070 766-766/com.example.jeet.calculator E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.jeet.calculator, PID: 766
java.lang.RuntimeException: Unable to start activity** ComponentInfo{com.example.jeet.calculator/com.example.jeet.calculator.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2338)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5299)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.jeet.calculator.MainActivity.onCreate(MainActivity.java:347)
at android.app.Activity.performCreate(Activity.java:5264)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1088)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2302)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5299)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：0)
我不确定，但我觉得你忘了在bEqual上做FindViewById