尝试在空对象引用上调用虚拟方法

Question

This is a script that counts how many times I pressed the button. When I run my code, I get this error:

01-17 19:49:18.670 27354-27354/com.example.ben.testapp
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ben.testapp, PID: 27354
java.lang.RuntimeException: Unable to instantiate activity
ComponentInfo{com.example.ben.testapp/com.example.ben.testapp.MainActivity}:
java.lang.NullPointerException: Attempt to invoke virtual method
'android.content.SharedPreferences
android.content.Context.getSharedPreferences(java.lang.String, int)'
on a null object reference
at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2515)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2723)
at android.app.ActivityThread.access$900(ActivityThread.java:172)
at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5832)
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:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual
method 'android.content.SharedPreferences
android.content.Context.getSharedPreferences(java.lang.String, int)'
on a null object reference
at
android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:184)
at
com.example.ben.testapp.MainActivity.numberOfClicks(MainActivity.java:89)
at com.example.ben.testapp.MainActivity.<init>(MainActivity.java:57)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1650)
at android.app.Instrumentation.newActivity(Instrumentation.java:1079)
at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2505)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2723) 
at android.app.ActivityThread.access$900(ActivityThread.java:172) 
at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:145) 
at android.app.ActivityThread.main(ActivityThread.java:5832) 
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:1399) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

I have recently added code meant to save the amount of times I have clicked the button.

It looks like this:

package com.example.ben.testapp;

import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();

            }
        });
    }

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


    private int clickNo = numberOfClicks();

    public void buttonClick(View view) {

        final TextView ClickCount = (TextView) findViewById(R.id.buttonClicks);

        clickNo = clickNo+1;

        if (clickNo == 10){
            Snackbar achievement10 = Snackbar.make(view,"acheivement unlocked: score of ten",Snackbar.LENGTH_LONG);
            achievement10.show();
        }

        if (clickNo == 20){
            Snackbar achievement20 = Snackbar.make(view,"acheivement unlocked: does your finger hurt?",Snackbar.LENGTH_LONG);
            achievement20.show();
        }

        if (clickNo == 50){
            Snackbar achievement50 = Snackbar.make(view,"acheivement unlocked: do you have a life?",Snackbar.LENGTH_LONG);
            achievement50.show();
        }

        String number = "" + clickNo ;

        ClickCount.setText(number);

        saveClicks();

    }

    private int numberOfClicks() {
        SharedPreferences prefs = getSharedPreferences("numberOf clicks",MODE_PRIVATE);
        return prefs.getInt("clicks", 0);
    }

    private void saveClicks(){
        SharedPreferences prefs = getSharedPreferences("numberOf clicks",MODE_PRIVATE);
        SharedPreferences.Editor editor = prefs.edit();
        editor.putInt("clicks", clickNo);
        editor.apply();
    }
}

Answer 1

Problem is in following line

private int clickNo = numberOfClicks();

You are initializing variable clickNo before MainActivity instance is created and therefore getSharedPreferences call is made on null object (your activity basecontext) reference.

You should declare variable without initializing it and then call numberOfClicks later on.

For instance:

private int clickNo;

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        clickNo = numberOfClicks();
        ...

Answer 2

You're assigning a null value to:

int clickNo = numberOfClicks();

The numberOfClicks() function returns null because you haven't ran saveClicks() yet, thus the preference value of clicks is null .