this keyword

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public class Car {
    private int topSpeed;

    Car() {
        topSpeed = 100;
    }
    
    public setTopSpeed(int topSpeed) {
        topSpeed = topSpeed
    }

    public int getTopSpeed() {
        return topSpeed;
    }
}

this keyword(2)

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public class Car {
    private int topSpeed;
    
    public setTopSpeed(int topSpeed) {
        this.topSpeed = topSpeed
    }

    public int getTopSpeed(int topSpeed) {
        return topSpeed;
    }
}

• Represents the current instance of the class in which it appears (means "my object")
• Clarifies scope

this keyword(3)

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public class Car {
    private int topSpeed;

    Car() {
        topSpeed = 100;
    }
    
    public setTopSpeed(int topSpeed) {
        this.topSpeed = topSpeed
    }

    public int getTopSpeed() {
        return topSpeed;
    }
}

Composition

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• A way to combine simple objects or data types into more complex ones
• "has a" relationship - car "has a" driver

Composition(2)

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What if every driver has not only name but points?

One way:

public class Car {
    ...
    private String driver;
    private int driverPoints;
    ...
}

Composition(3)

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Separate driver as a new class

public class Driver {

    private String name;
    private int points;

    public Driver(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public int getPoints() {
        return points;
    }

    public void setPoints(int points) {
        this.points = points;
    }

}

Composition(4)

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Change driver type in class Car

public class Car {

    private Driver driver;

    public Driver getDriver() {
        return driver;
    }

    public void setDriver(Driver driver) {
        this.driver = driver;
    }
}

References & Values

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Primitives vs References

• Primitives types are default Java types
  • int, double, boolean, char, float ...
  • The actual values are stored in the variable
• Reference types are arrays and objects
  • String, int[], Car, ...

References & Values(2)

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How Java stores primitives

• Variables are like fixed size cups
• Primitives are small enough that they just fit into the cup

References & Values(3)

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How Java stores objects

• Objects are too big to fit in a variable
  • Stored somewhere else
  • Variable stores a number(address) that locates the object

References & Values(4)

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References

• The object's location is called a reference

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Comparing objects

• == compares the references

Car nissan1 = new Car("Nissan GT-R");
Car nissan2 = new Car("Nissan GT-R");

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Comparing objects(2)

Car nissan1 = new Car("Nissan GT-R");
Car nissan2 = new Car("Nissan GT-R");

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Comparing objects(3)

• Using = updates the reference

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Comparing objects(4)

Car nissan1 = new Car("Nissan GT-R");
Car nissan2 = new Car("Nissan GT-R");
nissan1 = nissan2;

• About equals() - in the next lectures 

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Enum Types

• An enum type is a special data type that enables for a variable to be a set of predefined constants
• Often used for comparisons

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Enum Types(2)

• Definition - using the enum keyword
• Values are often written with capital letters

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
}

Usage example:

Day day1 = Day.MONDAY;
Day day2 = Day.TUESDAY;

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Enum Types(3)

Day day1 = Day.MONDAY;
Day day2 = Day.TUESDAY;
Day day3 = Day.MONDAY;

Comparison

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Enum Types(4)

• Enums are often used for comparison in switch statement

        switch (day) {
            case MONDAY:
                ...
                break;
                    
            case FRIDAY:
                ...
                break;
                         
            case SATURDAY: case SUNDAY:
                ...
                break;
                        
            default:
                ...
                break;
        }

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Enum Types(5)

Using enum for car color in Wacky Racers - green, red, blue, yellow

public enum CarColor {
    Yellow, Green, Red, Blue
}

public class Car {
    ...
    private CarColor color;

    public void setCarColor(CarColor color) {
        this.color = color;
    }

    public CarColor getCarColor() {
        return color;
    }
    ...
}

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static keyword

• Applies to fields and methods

• Means the field/method

  • Is defined for the class declaration

  • Is not unique for each instance 

     

     

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static field

public class Driver {
    ...
    public static int numOfDrivers = 0;
    ...
}

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static method

public class Driver {
    ...
    private static int numOfDrivers = 0;
    ...
    public static int getNumOfDrivers() {
        return numOfDrivers;
    }
}

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static keyword(2)

Where to increment numOfDrivers?