I'm confused how to deal with the xml code in Android Studio can't deal with Java code [on hold]

Question

I'm confused how to deal with the xml code in Android Studio can't deal with Java code here my code

xml class

 <manifest
        xmlns:android="http://schemas.android.com/apk/res/android"
        package="latihan11">
            <application
                android:allowBackup="true"
                android:icon="@mipmap/ic_launcher"
                android:label="@string/app_name"
                android:supportsRtl="true"
                android:theme="@style/AppTheme">
        <activity android:name=".apa">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"></action>
                <category
                    android:name="android.intent.category.LAUNCHER"></category>
            </intent-filter>
            </activity>
            </application>

List item

    </manifest>

this is my java class

package latihan11;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class apa extends Activity{
    public void onCreate(Bundle SaveIntanceState) {
        super.onCreate(SaveIntanceState);
        TextView tulisan = new TextView(this);
        tulisan.setText("hello world!");
        setContentView(tulisan);
    }
}

Answer 1

You are creating a TextView in Java code but not adding it to the layout.

To add the TextView in your layout, you first need to give id to the parent element in the layout in which you want to add the TextView .

Following is the XML and Java code that adds TextView to the layout.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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:id="@+id/test"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".TestActivity">

</android.support.constraint.ConstraintLayout>

import android.support.constraint.ConstraintLayout;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class TestActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Load UI
        setContentView(R.layout.activity_test);

        // Create TextView
        TextView tulisan = new TextView(this);

        // Set text of TextView
        tulisan.setText("hello world!");

        // Find the parent View to add TextView into
        ConstraintLayout parent = findViewById(R.id.test);

        // Add TextView in parent View
        parent.addView(tulisan);
    }
}

Also note that the statement of setContentView() comes before the findViewById() statement, else it will throw NullPointerException .