Key Concepts

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Java Objects

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java objects are instances of a Java class that contain state, in the form of instance fields, and behavior, in the form of methods. Java objects bring class into effect.

Java objects’ state and behavior

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, instances of a class are known as objects. Every object has state and behavior in the form of instance fields and methods respectively.

Class in Java with main() Method.

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java programs must contain one class with a main() method. main() method is invoked first. All other methods can be invoked via main() method.

Java main() method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, a program can have many classes, but one class must contain the main() method. Execution starts from main().

Constructor Method in Java.

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java classes contain a constructor method which is used to create instances of the class.

The constructor is named after the class. If no constructor is defined, a default empty constructor is used.

Java constructor

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, the constructor is a method whose name is the same as the class. The constructor is used to create instances of the class.

If no constructor is defined, a default empty constructor is used.

Instance Fields in Java.

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java instances contain state in the form of instance fields: variables declared within the scope of the instance.

Values for instance fields are assigned within the constructor method.

Java instance

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java instances are objects that are based on classes. For example, Bob may be an instance of the class Person.

Every instance has access to its own set of variables which are known as instance fields, which are variables declared within the scope of the instance. Values for instance fields are assigned within the constructor method.

Java Instances

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

A Java instance is created by the new keyword followed by invoking the constructor. Omitting the new keyword will cause an error.

For example, in this case, p is an instance of Person.

Person p = new Person();

Creating a new Class instance in Java

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, we use the new keyword followed by a call to the class constructor in order to create a new instance of a class.

The constructor can be used to provide initial values to instance fields.

Dot Notation in Java

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, dot notation . is used to access the value of fields and methods of an instance or Class.

The stucture used is instanceOrClassName.fieldOrMethodName.

Java dot notation

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java programming language, we use . to access the variables and methods of an object or a Class.

This is known as dot notation and the structure looks like this-

instanceOrClassName.fieldOrMethodName

Signature of a Java Method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java methods are defined with a method signature, which includes:

  • whether the method is public or private
  • what data type the method returns
  • what the method is called
  • what parameters (if any) the method takes

Java method signature

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, methods are defined with a method signature, which specifies the scope (private or public), return type, name of the method, and any parameters it receives.

Body of a Java Method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

The body of a Java method is enclosed by brackets after the method signature. Everything inside the body will be executed when the method is called.

The body of a Java method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, we use curly brackets {} to enclose the body of a method.

The statements written inside the {} are executed when a method is called.

Java Variables Inside a Method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java variables defined inside a method cannot be used outside the scope of that method.

Return Value of a Java Method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

Java methods can return a value. Any value that can be saved to a variable can be returned from a method using the return keyword. The type of variable returned must match the type specified in the method signature.

Returning info from a Java method

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

A Java method can return any value that can be saved in a variable. The value returned must match with the return type specified in the method signature.

The value is returned using the return keyword.

Parameters in Java

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In Java, parameters are variables that are declared in a method definition. They hold value and can be used in a method’s operations.

Method parameters in Java

public class Print { public void toPrint() { System.out.println("Hello World!"); } public static void main(String [] args) { Print p = new Print(); p.toPrint(); } } //In this example, `Print` is a class and `toPrint()` is a method, which is called by an object `p`.

In java, parameters are declared in a method definition. The parameters act as variables inside the method and hold the value that was passed in. They can be used inside a method for printing or calculation purposes.

In the example, a and b are two parameters which, when the method is called, hold the value 10 and 20 respectively.

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Java: Introduction to Classes
Lesson 1 of 2
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  1. 1
    All programs require one or more classes that act as a model for the world. For example, a program to track student test scores might have Student, Course, and Grade classes. Our real-world concer…
  2. 2
    The fundamental concept of object-oriented programming is the class. A class is the set of instructions that describe how an instance can behave and what information it contains. Java has pre-d…
  3. 3
    We create objects (instances of a class) using a constructor method. The constructor is defined within the class. Here’s the Car class with a constructor: public class Car { public Car() { /…
  4. 4
    Our last exercise ended with printing an instance of Store, which looked something like [email protected] The first part, Store, refers to the class, and the second part @6bc7c054 refers to the insta…
  5. 5
    We’ll use a combination of constructor method and instance field to create instances with individual state. We need to alter the constructor method because now it needs to access data we’re assig…
  6. 6
    Now that our constructor has a parameter, we must pass values into the method call. These values become the state of the instance. Here we create an instance, ferrari, in the main() method with “r…
  7. 7
    Objects are not limited to a single instance field. We can declare as many fields as are necessary for the requirements of our program. Let’s change Car instances so they have multiple fields. We…
  8. 8
    Java is an object-oriented programming language where every program has at least one class. Programs are often built from many classes and objects, which are the instances of a class. Classes def…
  1. 1
    In the last lesson, we created an instance of the Store class in the main method. We learned that objects have state and behavior: We have seen how to give objects state through instance fields…
  2. 2
    Remember our Car example from the last lesson? Let’s add a method to this Car called startEngine() that will print: Starting the car! Vroom! This method looks like: public void startEngine() {…
  3. 3
    Great! When we add the startEngine() method to the Car class, it becomes available to use on any Car object. We can do this by calling the method on the Car object we created, for example. Here …
  4. 4
    A method is a task that an object of a class performs. We mark the domain of this task using curly braces: {, and }. Everything inside the curly braces is part of the task. This domain is called t…
  5. 5
    We saw how a method’s scope prevents us from using variables declared in one method in another method. What if we had some information in one method that we needed to pass into another method? Sim…
  6. 6
    Earlier, we thought about a Savings Account as a type of object we could represent in Java. Two of the methods we need are depositing and withdrawing: public SavingsAccount{ double balance; p…
  7. 7
    Remember, variables can only exist in the scope that they were declared in. We can use a value outside of the method it was created in if we return it from the method. We return a value by u…
  8. 8
    When we print out Objects, we often see a String that is not very helpful in determining what the Object represents. In the last lesson, we saw that when we printed our Store objects, we would see …
  9. 9
    Great work! Methods are a powerful way to abstract tasks away and make them repeatable. They allow us to define behavior for classes, so that the Objects we create can do the things we expect them …

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