I'm trying to make a calculator app. First thing first I created a class that will take a string (my equation to solve) and change it from Infix to Postfix. The app is literally bear bones, but when I start it, it crashes immediately, and in the console I get no error. As anyone idea where might be the problem?
The manifest file:
<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.android.projectcalculator"> <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> XML file activity_main:
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/activity_main" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context="com.example.android.projectcalculator.MainActivity"> <TextView android:id="@+id/textCalc" android:layout_width="match_parent" android:layout_height="wrap_content" android:padding="10sp" android:layout_weight="1" android:text="Hello"/> </LinearLayout> The java file MainActivity:
package com.example.android.projectcalculator; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.TextView; import static android.R.attr.onClick; import com.example.android.projectcalculator.InfixToPostfix; public class MainActivity extends AppCompatActivity { public TextView calculationText; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); PrintMainScreen("hello you"); String s = InfixToPostfix.StartInfixToPostfix("A*(B+C)"); PrintMainScreen(s); } public void PrintMainScreen(String str) { TextView txview = (TextView)findViewById(R.id.textCalc); txview.setText(str); } } The Java file InfiToPostfix:
package com.example.android.projectcalculator; import java.util.Stack; public class InfixToPostfix{ //Varibili private private static String postfixOutput; private static Stack<Character> operatorStack; private static String infixInput; //Metodo per controlare se ho a che fare con l'operatore private static boolean IsOperator (char c) { return c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == ')' || c == '^'; } private static int OpratorPriority(Character operator1) { switch(operator1) { case '+': case '-': return 1; case '*': case '/': return 2; case '^': return 3; default: return 0; } } //Metodo Supremo public static String StartInfixToPostfix(String in) { //inizializzo variabili postfixOutput = ""; infixInput.equals(in); int lunghezza = infixInput.length(); operatorStack = new Stack<Character>(); //inizio il processo for (int i=0; i < infixInput.length(); i++) { //se non è un operatore ma un operando, lo aggiungo alla string di output if (!IsOperator(infixInput.charAt(i))) { postfixOutput += infixInput.charAt(i); } //Considero il caso in cui sia l'operatore ')' else if (infixInput.charAt(i) == ')') { //Inserisco nel postfix gli operatori fino a che lo sctack è vuoto o incontro una parentesi chiusa while (!operatorStack.isEmpty() && operatorStack.peek() != ')') { postfixOutput += (operatorStack.pop()); } //elimino la '(' se c'è if (!operatorStack.isEmpty()) { operatorStack.pop(); } } //considero il caso in cui ho un operatore che non sia ')' else { //questo while si attiva solo se (1) lo stack non è vuoto (2) l'elemento in cima allo stack non è '(' (3) se l'ultimo operatore ha grado minore while ( (!operatorStack.isEmpty()) && (operatorStack.peek() != '(') && (OpratorPriority(operatorStack.peek()) >= OpratorPriority(infixInput.charAt(i)))) { postfixOutput += operatorStack.pop(); } //aggiungo l'operatore a prescindere di ciò che ho fatto o non fatto con il cilo while operatorStack.push(infixInput.charAt(i)); } } //Alla fine del metodo rilascio il postfix return postfixOutput; } } Edit: Has anyone idea why I get error if everything in the InfixToPostfix class is not static?
Edit2: Now is giving me this error...
E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.android.projectcalculator, PID: 25981 Theme: themes:{} java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.projectcalculator/com.example.android.projectcalculator.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2450) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5461) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference at com.example.android.projectcalculator.InfixToPostfix.StartInfixToPostfix(InfixToPostfix.java:40) at com.example.android.projectcalculator.MainActivity.onCreate(MainActivity.java:20) at android.app.Activity.performCreate(Activity.java:6251) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2403) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5461) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
infixInputis null.