Making a java interface

Question

I am learning to write a interface in java in android studios but for some reason my app always says "Unfortunately myApp has stopped working" or something of the like

this is what i have:

package com.name.javainterface;

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.RelativeLayout;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

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

    RelativeLayout myLayout = new RelativeLayout(this);
    Button redButton = new Button(this);

    myLayout.addView(redButton);

    setContentView(R.layout.myLayout);

    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);
    }
}

How come my layout is causing my app to stop functioning ? when i plug my phone in and run the app it doesn't play and says "error: cannot find symbol variable myLayout" which doesn't make any sense because i made the layout before calling it so why doesn't it know what myLayout is?

Answer 1

Don't add default views like button, textview, edittext, etc. programmatically until it is required. You have declared setContentView() multiple times also. Plus, the error: "cannot find symbol variable myLayout" is generated because you haven't created a layout file named "myLayout.xml" in layout directory. Here is the fixed code:

MainActivity.java :

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button redButton=(Button) findViewById(R.id.button_red);
    }

}

activity_main.xml :

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/button_red"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="I am red button"/>

</RelativeLayout>

Also, looking at your programming style, it seems you are new to android. So please watch these set of tutorials: https://www.youtube.com/watch?v=hzrGAZnMOMQ&list=PL1q3ROAofjeOUwh7lPBnGbg__DUodwLN7