null object referenceの解消方法

android studio でタブでフラグメントを切り替えるプログラムを書いているのですが、以下のようなエラーが出て実行できません。
どなたか良い解決方法はありませんでしょうか？

java
1package com.example.you_1_low.tabtest;
2
3import android.os.Build;
4import android.support.annotation.RequiresApi;
5import android.support.v4.view.ViewPager;
6import android.support.v7.app.AppCompatActivity;
7import android.os.Bundle;
8import android.view.View;
9import android.widget.TextView;
10
11public class MainActivity extends AppCompatActivity {
12    TextView camera, chat, status, call;
13    ViewPager viewPager;
14    PagerViewAdapter pagerViewAdapter;
15
16
17    @Override
18    protected void onCreate(Bundle savedInstanceState) {
19        super.onCreate(savedInstanceState);
20        setContentView(R.layout.activity_main);
21        camera = (TextView)findViewById(R.id.camera);
22        camera = (TextView)findViewById(R.id.chat);
23        camera = (TextView)findViewById(R.id.status);
24        camera = (TextView)findViewById(R.id.call);
25        viewPager = (ViewPager)findViewById(R.id.fragment_container);
26
27        pagerViewAdapter = new PagerViewAdapter(getSupportFragmentManager());
28
29        viewPager.setAdapter(pagerViewAdapter);
30
31        camera.setOnClickListener(new View.OnClickListener(){
32
33            @Override
34            public void onClick(View v) {
35
36
37                viewPager.setCurrentItem(0);
38            }
39        });
40
41        chat.setOnClickListener(new View.OnClickListener(){
42
43            @Override
44            public void onClick(View v) {
45
46
47                viewPager.setCurrentItem(1);
48            }
49        });
50
51        status.setOnClickListener(new View.OnClickListener(){
52
53            @Override
54            public void onClick(View v) {
55
56
57                viewPager.setCurrentItem(2);
58            }
59        });
60
61        call.setOnClickListener(new View.OnClickListener(){
62
63            @Override
64            public void onClick(View v) {
65
66
67                viewPager.setCurrentItem(3);
68            }
69        });
70
71        viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener(){
72
73            @Override
74            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
75
76            }
77
78            @Override
79            public void onPageSelected(int position) {
80
81                onChangeTab(position);
82
83            }
84
85            @Override
86            public void onPageScrollStateChanged(int state) {
87
88            }
89        });
90
91
92    }
93
94    @RequiresApi(api = Build.VERSION_CODES.M)
95    private void onChangeTab(int position) {
96        if(position == 0)
97        {
98            camera.setTextSize(20);
99            camera.setTextColor(getColor(R.color.bright_color));
100
101            chat.setTextSize(20);
102            chat.setTextColor(getColor(R.color.light_color));
103
104            status.setTextSize(20);
105            status.setTextColor(getColor(R.color.bright_color));
106
107            call.setTextSize(20);
108            call.setTextColor(getColor(R.color.bright_color));
109        }
110
111        if(position == 1)
112        {
113            camera.setTextSize(25);
114            camera.setTextColor(getColor(R.color.light_color));
115
116            chat.setTextSize(25);
117            chat.setTextColor(getColor(R.color.light_color));
118
119            status.setTextSize(25);
120            status.setTextColor(getColor(R.color.bright_color));
121
122            call.setTextSize(25);
123            call.setTextColor(getColor(R.color.bright_color));
124        }
125
126        if(position == 2)
127        {
128            camera.setTextSize(25);
129            camera.setTextColor(getColor(R.color.light_color));
130
131            chat.setTextSize(25);
132            chat.setTextColor(getColor(R.color.light_color));
133
134            status.setTextSize(25);
135            status.setTextColor(getColor(R.color.bright_color));
136
137            call.setTextSize(25);
138            call.setTextColor(getColor(R.color.bright_color));
139        }
140
141        if(position == 3)
142        {
143            camera.setTextSize(25);
144            camera.setTextColor(getColor(R.color.light_color));
145
146            chat.setTextSize(25);
147            chat.setTextColor(getColor(R.color.light_color));
148
149            status.setTextSize(25);
150            status.setTextColor(getColor(R.color.bright_color));
151
152            call.setTextSize(25);
153            call.setTextColor(getColor(R.color.bright_color));
154        }
155    }
156}
157

エラー詳細
2018-10-29 23:22:11.113 4732-4732/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.you_1_low.tabtest, PID: 4732
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.you_1_low.tabtest/com.example.you_1_low.tabtest.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.you_1_low.tabtest.MainActivity.onCreate(MainActivity.java:41)
at android.app.Activity.performCreate(Activity.java:6679)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

行動規範の内容に同意します

回答1件

ベストアンサー

エラーが出ているのは、値が代入されていない変数chatを参照しているからです。このコードだと、statusとcallという変数も同じ問題を抱えています。

onCreate()の序盤で

java
1        camera = (TextView)findViewById(R.id.camera);
2        camera = (TextView)findViewById(R.id.chat);
3        camera = (TextView)findViewById(R.id.status);
4        camera = (TextView)findViewById(R.id.call);