package com.example.myapplication;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;
public class MainActivity extends Activity {
private Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
try {
button = (Button) findViewById(R.id.btnLogin);
}
catch (Exception e) {
e.printStackTrace();
Log.e("Exception raised", e.toString());
}
}
}
Look at the line 14 to line 20, we have put a try-catch statement here. The purpose of this statement is to anticipate if an exception will
be raised inside the try statement, in this example at line 15. This exception will be raised for example if findViewById(R.id.btnLogin) return
object not a Button type.
package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public class Child extends Father {
}
From the code above Child is called a subclass and Father is called a superclass.
There is time you need to cast a superclass to its subclass, let's see one example :
public void createPeople() {
ArrayList<Father> list = new ArrayList<Father>();
Father father = new Father();
Child child = new Child();
list.add(father);
list.add(child);
int size = list.size();
for(int i = 0; i < size ; i++) {
Father father1 = list.get(i);
if(father1 instanceof Child){
Child child1 = (Child) father1;
}
}
}
package com.example.myapplication;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends Activity {
private Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.btnLogin);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
}
});
}
}
In android code above, at line 14, findViewById(R.id.btnLogin); return value is a type of View (look at reference here ), but then is casted to type Button. Since Button extends View , see reference here , this return value of findViewById could be a Button type. But you should make sure that the id R.id.btnLogin is referring to is the right Button widget in layout xml file. If it does not correctly refer to Button widget in xml layout, a ClassCastException will be raised that will make your app force close.package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public class Child extends Father {
}
It is not valid to extends from two classes like below :
package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public class Child extends Father, Mother {
}
And it is not valid also to write extends two times like below :
package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public class Child extends Father extends Mother {
}
But like in the real world, you need the child inherit properties from mother and father (not just from mother only or father only). Now, I want a child has white skin like his mother and black hair like his father. Okay, that is why we have interface. Let's do that.package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public interface IMother {
public void setSkinWhite();
}
package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public interface IFather {
public void setHairBlack();
}
Now let's make Child that inherit both IMother and IFather properties.
package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public class Child implements IMother, IFather {
@Override
public void setHairBlack() {
}
@Override
public void setSkinWhite() {
}
}
package com.example.myapplication;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends Activity {
private Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
}
});
package com.example.myapplication;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends Activity {
private Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.btnLogin);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
}
});
}
}
In the code above button variable has been initialized by findViewById(R.id.btnLogin); and it
is safe now to call button.setOnClickListener, no NullPointerException will be raised.
package com.example.myapplication.alert;
/**
* Created by SONY on 17/10/2016.
*/
public class Student {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Student student = new Student();
student.setName("Robert");
String name = student.getName();
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.myapplication">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name="com.example.myapplication.MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
package com.example.myapplication;
import android.app.Activity;
import android.os.Bundle;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
See that MainActivity extends Activity, Activity is a class from Android Library SDK, you don't write it, the google team write it for you, you just extends it. That activity calls onCreate method and set content to your page in line setContentView(R.layout.activity_main); That is why you see layout activity_main.xml is shown to you. You put activity_main.xml in layout folder under res folder separated from java files, to make easier for you to manage source code. see : explore android studio folders and files. If you have understand this basic concept you are ready for next lessons. keep in mind that everything is object.