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For the JAVA file:

public class UserDataInputActivity extends AppCompatActivity

 @Override
 protected void onCreate(Bundle savedInstanceState)
 
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_user_data_input);
 

 public void onGenericMenuClick(View view)
 
 ImageView pastTests = (ImageView) findViewById(R.id.pasttests);
 final Intent intent1 = new Intent(this, PastDiagnosticResult.class);
 ImageView userData = (ImageView) findViewById(R.id.myinfo);
 final Intent intent2 = new Intent(this, UserDataInputActivity.class);
 ImageView currentTests = (ImageView) (findViewById(R.id.currenttest));
 final Intent intent3 = new Intent(this, CurrentDiagnosticResultActivity.class);

 pastTests.setOnClickListener(new OnClickListener()
 public void onClick(View view)
 
 startActivity(intent1);
 );
 userData.setOnClickListener(new OnClickListener()
 public void onClick(View view)
 
 startActivity(intent2);
 );
 currentTests.setOnClickListener(new OnClickListener()
 public void onClick(View view)
 
 startActivity(intent3);
 );
 

I essentially have 3 icons (imageViews) on the bottom of my screen. My objective: to navigate to different screens as I click the different images on the menu. However, my java code does not work correctly. Although it runs, it crashes after I try to use Intents 2 and 3 consecutively. Is there a better way to do this?

Corresponding LOGCAT Error Output:

2018-11-11 02:22:02.224 7638-7638/com.example.owner.introductoryapplication E/AndroidRuntime: FATAL EXCEPTION: main
 Process: com.example.owner.introductoryapplication, PID: 7638
 java.lang.IllegalStateException: Could not execute method for android:onClick
 at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:390)
 at android.view.View.performClick(View.java:6312)
 at android.view.View$PerformClick.run(View.java:24802)
 at android.os.Handler.handleCallback(Handler.java:790)
 at android.os.Handler.dispatchMessage(Handler.java:99)
 at android.os.Looper.loop(Looper.java:169)
 at android.app.ActivityThread.main(ActivityThread.java:6521)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
 Caused by: java.lang.reflect.InvocationTargetException
 at java.lang.reflect.Method.invoke(Native Method)
 at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385)
 at android.view.View.performClick(View.java:6312) 
 at android.view.View$PerformClick.run(View.java:24802) 
 at android.os.Handler.handleCallback(Handler.java:790) 
 at android.os.Handler.dispatchMessage(Handler.java:99) 
 at android.os.Looper.loop(Looper.java:169) 
 at android.app.ActivityThread.main(ActivityThread.java:6521) 
 at java.lang.reflect.Method.invoke(Native Method) 
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
 at com.example.owner.introductoryapplication.CurrentDiagnosticResultActivity.onGenericMenuClick(CurrentDiagnosticResultActivity.java:28)
 at java.lang.reflect.Method.invoke(Native Method) 
 at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385) 
 at android.view.View.performClick(View.java:6312) 
 at android.view.View$PerformClick.run(View.java:24802) 
 at android.os.Handler.handleCallback(Handler.java:790) 
 at android.os.Handler.dispatchMessage(Handler.java:99) 
 at android.os.Looper.loop(Looper.java:169) 
 at android.app.ActivityThread.main(ActivityThread.java:6521) 
 at java.lang.reflect.Method.invoke(Native Method) 
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Edit #2 - XML File:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:background="#2E2A27"
 tools:context=".UserDataInputActivity">

 <ImageView
 android:id="@+id/header"
 android:src="@drawable/companyicon"
 android:layout_width="fill_parent"
 android:layout_height="50dp"
 android:background="#211F1E"
 tools:layout_editor_absoluteY="4dp" />

 <TextView
 android:id="@+id/textView"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_alignParentTop="true"
 android:layout_centerHorizontal="true"
 android:layout_marginTop="82dp"
 android:text="@string/PersonalInfo"
 android:textSize="30sp"
 style="@style/TextHeaderAttr"/>

 <LinearLayout
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_alignParentBottom="true"
 android:weightSum="3"
 android:background="#211F1E">

 <ImageView
 android:src="@drawable/diagnosisicon"
 android:id="@+id/pasttests"
 style="@style/IconStyle"
 android:layout_width="0dp"
 android:layout_height="40dp"
 android:layout_weight="1"
 android:layout_gravity="center"
 android:layout_marginBottom="5dp"
 android:layout_marginTop="5dp"
 android:onClick="onGenericMenuClick"/>
 <ImageView
 android:src="@drawable/profileicon"
 android:id="@+id/myinfo"
 style="@style/IconStyle"
 android:tint="#A9A9A9"
 android:layout_width="0dp"
 android:layout_height="50dp"
 android:layout_weight="1"
 android:layout_gravity="center"
 android:layout_marginBottom="5dp"
 android:layout_marginTop="5dp"
 android:onClick="onGenericMenuClick"/>
 <ImageView
 android:src="@drawable/resultsicon"
 android:id="@+id/currenttest"
 style="@style/IconStyle"
 android:layout_width="0dp"
 android:layout_height="40dp"
 android:layout_weight="1"
 android:layout_gravity="center"
 android:layout_marginBottom="5dp"
 android:layout_marginTop="5dp"
 android:onClick="onGenericMenuClick"/>
 </LinearLayout>

 <!-- CONTENT -->

</RelativeLayout>

You are not using onGenericMenuClick() as you should.

This is the common onClick() for all 3 views so you don't define inside it new listeners.

Change to this:

public void onGenericMenuClick(View view) 
 Intent intent;

 if(v.getId() == R.id.pasttests) 
 intent = new Intent(this, PastDiagnosticResult.class);
 else if (v.getId() == R.id.myinfo) 
 intent = new Intent(this, UserDataInputActivity.class);
 else if (v.getId() == R.id.currenttest) 
 intent = new Intent(this, CurrentDiagnosticResultActivity.class);
 

 startActivity(intent);

This is a case, where you should use Fragments instead Activities.

1.Replace three Activities with fragments.

2.Use FragmentManager to switch between fragments.

UserDataInputFragment userDataInputFragment = UserDataInputFragment.newInstance("","");

getSupportFragmentManager().beginTransaction()
 .replace(R.id.fragment, userDataInputFragment, "userDataInputFragment")
 .commit();
 //here fragment in R.id.fragment is the container of Fragments