package com.example.android.kalkulatron;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;
import java.text.DecimalFormat;
public class MainActivity extends AppCompatActivity {
TextView nomor1 = (TextView) findViewById(R.id.Nomor1);
TextView nomor2 = (TextView) findViewById(R.id.Nomor2);
TextView tanda = (TextView) findViewById(R.id.tanda);
Double no1 = Double.parseDouble(nomor1.getText().toString());
Double no2 = Double.parseDouble(nomor2.getText().toString());
Double sum;
boolean Ceksum;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
private void Fsatu(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "1");
} else {
nomor1.setText("1");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "1");
}
}
private void Fdua(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "2");
} else {
nomor1.setText("2");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "1");
}
}
private void Ftiga(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "3");
} else {
nomor1.setText("2");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "2");
}
}
private void Fempat(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "4");
} else {
nomor1.setText("4");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "4");
}
}
private void Flima(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "5");
} else {
nomor1.setText("5");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "5");
}
}
private void Fenam(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "6");
} else {
nomor1.setText("6");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "6");
}
}
private void Ftujuh(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "7");
} else {
nomor1.setText("7");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "7");
}
}
private void Fdelapan(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "8");
} else {
nomor1.setText("8");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "8");
}
}
private void Fsembilan(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "9");
} else {
nomor1.setText("9");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "9");
}
}
private void Ftitik(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + ".");
} else {
nomor1.setText(".");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + ".");
}
}
private void Fnol(View view) {
if (tanda.getText().length() == 0) {
if (Ceksum = false){
nomor1.setText(nomor1 + "0");
} else {
nomor1.setText("0");
Ceksum = false;
}
} else {
nomor2.setText(nomor2 + "0");
}
}
public void Ftambah(View view) {
tanda.setText("+");
}
public void Fkurang(View view) {
tanda.setText("-");
}
public void Fkali(View view) {
tanda.setText("X");
}
public void Fbagi(View view) {
tanda.setText("÷");
}
public void Fclear(View view) {
tanda.setText("");
nomor1.setText("");
nomor2.setText("");
}
public void Fequals(View view) {
DecimalFormat df = new DecimalFormat("#.##");
if (nomor2.getText().length() != 0) {
if (tanda.getText() == "+") {
sum = no1 + no2;
sum = Double.valueOf(df.format(sum));
nomor1.setText(""+sum);
} else if (tanda.getText() == "-") {
sum = no1 - no2;
sum = Double.valueOf(df.format(sum));
nomor1.setText(""+sum);
} else if (tanda.getText() == "X") {
sum = no1 * no2;
sum = Double.valueOf(df.format(sum));
nomor1.setText(""+sum);
} else if ((tanda.getText() == "÷") && (nomor2.getText() != "0")) {
sum = no1 / no2;
sum = Double.valueOf(df.format(sum));
nomor1.setText(""+sum);
}
tanda.setText("");
nomor2.setText("");
} else {
Toast toast = new Toast(this);
toast.makeText(getApplicationContext(),
"Wrong input, repeat!", Toast.LENGTH_SHORT).show();
tanda.setText("");
nomor2.setText("");
nomor1.setText("");
}
Ceksum = true;
}
}
这是我的Android计算器原型，但是当我在手机上测试它时它会不断崩溃。我这样做只是为了试验和错误，在这里我坚持这个。任何人都能找到我犯的错误吗？非常感谢你提前。 （抱歉英文不好）
05-04 14:24:07.937 5391-5391/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.kalkulatron, PID: 5391
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.kalkulatron/com.example.android.kalkulatron.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2290)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442)
at android.app.ActivityThread.access$800(ActivityThread.java:156)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:211)
at android.app.ActivityThread.main(ActivityThread.java:5373)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:68)
at android.support.v7.app.AppCompatDelegateImplV7.<init>(AppCompatDelegateImplV7.java:145)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:28)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:42)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:186)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:168)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:508)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:180)
at com.example.android.kalkulatron.MainActivity.<init>(MainActivity.java:13)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2280)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442)
at android.app.ActivityThread.access$800(ActivityThread.java:156)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:211)
at android.app.ActivityThread.main(ActivityThread.java:5373)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)
答案 0 :(得分：1)
像这样改变
Double sum;
boolean Ceksum;
TextView nomor1;
TextView nomor2;
TextView tanda;
Double no1 = 0d;
Double no2 = 0d;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
nomor1 = (TextView) findViewById(R.id.Nomor1);
nomor2 = (TextView) findViewById(R.id.Nomor2);
tanda = (TextView) findViewById(R.id.tanda);
no1 = Double.parseDouble(nomor1.getText().toString());
no2 = Double.parseDouble(nomor2.getText().toString());
}