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5. Constructors in Java

• The Java Tutorials - Providing Constructors for Your Classes.
• Java Constructors.
• Constructors in Java.

Create a new Eclipse project named task05. Use a package named ua.khpi.oop.your_first_name.task05 for all created classes.

1) Add a class called ConstructorEx to your package. Give it a field named array:

class ConstructorEx {
	int[] array;
}

2) Add three constructors to your class:

ConstructorEx(int length) {
	// TODO: Initialize the field "array" by creating a new array using the
	//       specified "length".
}

ConstructorEx(int[] srcArray) {
	// TODO: Initialize the "array" field with the reference
	//       to the specified "srcArray".
}

ConstructorEx(int[] srcArray, boolean copyArray) {
	// TODO: Initialize the "array" field by reference to the specified "srcArray"
	//       or by copying the contents of the "srcArray" depending on the "copyArray" flag.
}

3) Use the following class Main to test the implemented constructors of class ConstructorEx.

public class Main {

	public static void main(String[] args) {
		System.out.println("Begin...");

		ConstructorEx a = new ConstructorEx(3);
		System.out.println(Arrays.toString(a.array));
		System.out.println("---");

		int[] array = new int[5];

		ConstructorEx b = new ConstructorEx(array);
		Arrays.fill(array, 1);
		System.out.println(Arrays.toString(array));
		System.out.println(Arrays.toString(b.array));
		System.out.println("---");

		Arrays.fill(array, 2);
		System.out.println(Arrays.toString(array));
		System.out.println(Arrays.toString(b.array));
		System.out.println("---");

		ConstructorEx c = new ConstructorEx(array, true);
		Arrays.fill(array, 3);
		System.out.println(Arrays.toString(array));
		System.out.println(Arrays.toString(c.array));
		System.out.println("---");

		ConstructorEx d = new ConstructorEx(array, false);
		Arrays.fill(array, 4);
		System.out.println(Arrays.toString(array));
		System.out.println(Arrays.toString(d.array));

		System.out.println("Done.");
	}

}

You should obtain the following result:

Begin...
[0, 0, 0]
---
[1, 1, 1, 1, 1]
[1, 1, 1, 1, 1]
---
[2, 2, 2, 2, 2]
[2, 2, 2, 2, 2]
---
[3, 3, 3, 3, 3]
[2, 2, 2, 2, 2]
---
[4, 4, 4, 4, 4]
[4, 4, 4, 4, 4]
Done.

Extra

• The Java Tutorials - Providing Constructors for Your Classes.
• The Java Tutorials - Using the this Keyword.
• The Java Tutorials - Controlling Access to Members of a Class.

1) Fix the compilation errors of the following program.

class One {
	private int x;

	void One() {
		System.out.println("One::One()");
		this(999);
	}

	One(int x) {
		System.out.println("One::One(int)");
		x = this.x;
	}

	private One getInstance() {
		return null;
	}
}

2) Use the following Main class to test the implementation of the One class.

public class Main {

	public static void main(String[] args) {
		One a = new One();
		System.out.println(a.x);
		One b = a.getInstance();
		System.out.println(a == b);
	}

}

3) If necessary, сhange the implementation of the One class to obtain the following results.

One::One(int)
One::One()
999
true

Hint: Do not specify return type in constructor declaration.

Hint: If present, the invocation of another constructor must be the first line in the constructor.

Hint: Do not use private members to access them outside their class.

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