应用程序未在Android Studio模拟器中运行

Question

Started learning android studios and when I try to run a simple app, I keep on getting the following errors

Error: Cannot find symbol variable toolbar

Execution Failed for task :app:CompileDebugJavawithJavac

Could someone please help me.

Below is the code for my activity_main.xml

<?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:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.mohamedali.helloworld.MainActivity">


<TextView
    android:layout_width="wrap_content"
    android:layout_height="16dp"
    android:text="Mohamed First App!!"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

Also this is the actual code

package com.example.mohamedali.helloworld;

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;

public class MainActivity extends AppCompatActivity {

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

    }catch (Exception e){
        System.out.println("Problem is: " + e.getMessage());
        }
    }

}

Answer 1

There is a great tutorial about using the App Toolbar here

You will need to start by making a toolbar layout.

Then include it in your layout (in your case activity_main.xml)

Then you can use the toolbar with

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

Answer 2

Comment out or remove these 2 lines in MainActivity

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

Answer 3

You currently have no reference to the @id+="toolbar" in your resources for your toolbar object to reference to.

Simply, you would need to add a toolbar in your xml with a line of reference like:

android:id="@+id/toolbar"

This can then be referenced by the "R.id.toolbar" in your line of code:

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);