Lesson 0.2: Fields, Constructors, and Methods

A class becomes useful when it defines what its objects remember, how they start, and what they can do. Java names those parts fields, constructors, and methods.

If words such as private, double, public, this, new, or void are still unfamiliar, use the Java keyword reference from Lesson 0.1 while you work through the examples below.

The house blueprint still helps:

• A blueprint lists features such as an address and paint color. A class declares fields.
• A construction process turns the plan into one finished house. A constructor prepares one new object.
• A completed house supports actions such as opening its door. Methods define actions for an object.

Now connect each part to Java.

Fields Store Object State

private String name;
private double power;

A field is a variable that belongs to an object. Every RobotArm object gets its own name and power. Together, those values form the object's state.

private means that code outside RobotArm cannot change these fields directly. Unit 13 explains that protection as encapsulation. For now, notice that the class controls its own values.

A Constructor Prepares a New Object

public RobotArm(String name) {
    this.name = name;
    this.power = 0.0;
}

A constructor has the same name as its class and no return type. Java runs it when new creates an object.

Read the first assignment carefully:

• this.name means the name field on the object being constructed.
• The plain name on the right means the argument supplied by the caller.

RobotArm arm = new RobotArm("main arm");

new creates the object. The constructor receives "main arm", stores that value in the object's field, and starts its power at zero.

Methods Define Behavior

public void move(double requestedPower) {
    power = requestedPower;
}

public void stop() {
    move(0.0);
}

A method is a named section of behavior. move receives a parameter and changes the object's state. stop calls another method on the same object.

The word void means the method does not return a value. A method that reports information can return a value instead:

public double getPower() {
    return power;
}

Annotated Code

One complete class

public class RobotArm {
  private String name;
  private double power;

  public RobotArm(String name) {
      this.name = name;
      this.power = 0.0;
  }

  public void move(double requestedPower) {
      power = requestedPower;
  }

  public double getPower() {
      return power;
  }
}

Python uses self where Java uses this. It also uses __init__ for object setup. The spelling differs, but both versions store state on one object and provide methods that operate on it.

Terms to Keep

Term	Meaning
Field	A variable stored on an object.
Constructor	Code that prepares an object created with new.
Parameter	A named input received by a constructor or method.
Argument	The value supplied by the caller.
this	The current object whose code is running.
return	Send a result back to the caller.

Check Your Understanding

When does the RobotArm constructor run?

It runs when Java evaluates new RobotArm(...).

Why does stop call move instead of assigning power directly?

The class keeps one power-setting path. Any later validation added to move also applies when stop calls it.

What does this.name select?

It selects the name field on the current object.

Where You Will Use This

Lesson 13.1 uses fields and public methods to give a mechanism a safe interface.

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