Android create object class

Create complex object at run-time with Builder pattern

May 25, 2017 · 3 min read

A design pattern is a well described solution to a common software problem. It also helps to prevent subtle issues that can cause major problems and improves code readability with the patterns.

The Builder is a creational pattern. With Builder pattern we can separate the construction of a complex object from its representation and build different objects using same construction process. Builder pattern is more likely implemented using method cascading.

As mentioned in the Gang of four —

“Separate the construction of a complex object from its representation so that the same construction process can create different representations.”

Where to use?

Following are the conditions which we can easily handle using builder design pattern-

1. When multiple representation of objects are required.

2 . Too many argument to pass from client.

3. Object creation contains optional parameter.

What problem it solve?

We all know each class has a constructor in Java and if we don’t explicitly declare a constructor of a class then compiler builds a default constructor for that class. We can also create parameterized constructor which can take different parameters required to create an object.

Problem starts when an object can be created with lot of parameters and some of them may be mandatory and others may be optional.

In general we pass the NULL values for the optional parameters or overload more constructors to create a desired object of a class. Situation become more error prone when many fields has same type and we confused with their order while passing the values to the constructor.

To create an object of Employee class, we must pass all the values of parameters to its constructor. It will look like this:

Now what if only id, firstName and lastName are mandatory and rest fields are optional then object generation looks like-

Any like many more constructor…

The problem with above approach is that the object state will be inconsistent during building. Here builder design pattern will help you to achieve the goal.

How builder works

Below are the steps required to create a builder pattern:

• Create a builder class with all the required fields.
• Builder class should have a public constructor with all the required parameters.
• Create methods to get the values of optional parameters. These methods should return the same builder object after setting the optional attribute.
• Finally provide a build() method in the builder class which will return the desired object.

Here is the sample builder pattern example code where we have Employee class and EmployeeBuilder class to build it.

You can get the complete example code from GitHub.

And below is the way, we will use the EmployeeBuilder in our code:

Notice that Employee class has only no public constructor and buildEmployee() method in the EmployeeBuilder class is the only way to create Employee object.

Examples in JDK

Builder pattern is also used in Java classes like —

Conclusion

Using builder design patter we can have more maintainable code if number of fields required to create object is increased. It also reduce the chance to pass invalid values because of explicit optional method calls.

Thanks for reading. To help others please click ❤ to recommend this article if you found it helpful.

Stop of being selfish, spread the knowledge

Stay tuned for upcoming articles. For any quires or suggestions, feel free to hit me on Twitter Google+

Check out my blogger page for more interesting topics on Software development.

Источник

How To Create New Java Class In Android Studio

In android development, the java class is commonly used to implement business logic processes such as database access, send emails, send short messages etc. We always use one java class to encapsulate some related functions.

So we can think java class as the Model role in the MVC pattern. This example will show you how to create a new java class in android studio, and we will create a SMSBean java class that will implement short message send and receives functions.

1.How To Create New Java Class In Android Studio.

1. Launch Android Studio, create a new project. You can read the article How To Create New Android Studio Project to learn more.
2. After that right-click the default package in the android studio Project View’s Android subview, then click the New —> Java Class menu item.
3. You can also click the New —> Package menu item to create a new package or sub-package under exist package. In this example, we create a sub-package model under the existing com.dev2qa.example package.
4. Then there will popup a dialog that will let you input java class-related information. Input class name SMSBean in the Name input text box, select Class in the Kind drop-down list, do not input any superclass, do not implement any interface. Input package name value such as com.dev2qa.example.model. Click the OK button.
5. After clicking the OK button, the java class SMSBean has been created successfully. We will add two methods in it, one is sendSMS(), the other is receiveSMS(), please see code comments for detail.

2. Java Class SMSBean Usage Example.

This example will use SMSBean as a short messages manager object, it has two methods sendSMS() and receiveSMS().

The user interface of this example is not complex, there will have two buttons on the screen one is used to send short messages, the other is used to receive short messages. When you click any of the two buttons, an AlertDialog will pop up showing some messages.

If you can not watch the above video, you can see it on the youtube URL https://youtu.be/VhM3qIQkuD8

So if you want to run this example, you should create a new activity and layout XML files to implement the user interface. You can read the below articles if you do not know how to create activity and use event listener.

Источник

Create complex object at run-time with Builder pattern

May 25, 2017 · 3 min read

A design pattern is a well described solution to a common software problem. It also helps to prevent subtle issues that can cause major problems and improves code readability with the patterns.

The Builder is a creational pattern. With Builder pattern we can separate the construction of a complex object from its representation and build different objects using same construction process. Builder pattern is more likely implemented using method cascading.

As mentioned in the Gang of four —

“Separate the construction of a complex object from its representation so that the same construction process can create different representations.”

Where to use?

Following are the conditions which we can easily handle using builder design pattern-

1. When multiple representation of objects are required.

2 . Too many argument to pass from client.

3. Object creation contains optional parameter.

What problem it solve?

We all know each class has a constructor in Java and if we don’t explicitly declare a constructor of a class then compiler builds a default constructor for that class. We can also create parameterized constructor which can take different parameters required to create an object.

Problem starts when an object can be created with lot of parameters and some of them may be mandatory and others may be optional.

In general we pass the NULL values for the optional parameters or overload more constructors to create a desired object of a class. Situation become more error prone when many fields has same type and we confused with their order while passing the values to the constructor.

To create an object of Employee class, we must pass all the values of parameters to its constructor. It will look like this:

Now what if only id, firstName and lastName are mandatory and rest fields are optional then object generation looks like-

Any like many more constructor…

The problem with above approach is that the object state will be inconsistent during building. Here builder design pattern will help you to achieve the goal.

How builder works

Below are the steps required to create a builder pattern:

• Create a builder class with all the required fields.
• Builder class should have a public constructor with all the required parameters.
• Create methods to get the values of optional parameters. These methods should return the same builder object after setting the optional attribute.
• Finally provide a build() method in the builder class which will return the desired object.

Here is the sample builder pattern example code where we have Employee class and EmployeeBuilder class to build it.

You can get the complete example code from GitHub.

And below is the way, we will use the EmployeeBuilder in our code:

Notice that Employee class has only no public constructor and buildEmployee() method in the EmployeeBuilder class is the only way to create Employee object.

Examples in JDK

Builder pattern is also used in Java classes like —

Conclusion

Using builder design patter we can have more maintainable code if number of fields required to create object is increased. It also reduce the chance to pass invalid values because of explicit optional method calls.

Thanks for reading. To help others please click ❤ to recommend this article if you found it helpful.

Stop of being selfish, spread the knowledge

Stay tuned for upcoming articles. For any quires or suggestions, feel free to hit me on Twitter Google+

Check out my blogger page for more interesting topics on Software development.

Источник

Creating Objects

As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:

The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.

Each of these statements has three parts (discussed in detail below):

1. Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
2. Instantiation: The new keyword is a Java operator that creates the object.
3. Initialization: The new operator is followed by a call to a constructor, which initializes the new object.

Declaring a Variable to Refer to an Object

Previously, you learned that to declare a variable, you write:

This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.

You can also declare a reference variable on its own line. For example:

If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.

A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne , plus a reference pointing to nothing):

Instantiating a Class

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.

The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.

The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:

The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:

This statement will be discussed in the next section.

Initializing an Object

Here’s the code for the Point class:

This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:

The result of executing this statement can be illustrated in the next figure:

Here’s the code for the Rectangle class, which contains four constructors:

Each constructor lets you provide initial values for the rectangle’s origin, width, and height, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

This calls one of Rectangle ‘s constructors that initializes origin to originOne . Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object—an object can have multiple references to it, as shown in the next figure:

The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:

The Rectangle constructor used in the following statement doesn’t take any arguments, so it’s called a no-argument constructor:

All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent’s no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor ( Object does have one), the compiler will reject the program.

Источник