Chapter 5

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Chapter 5

Defining Classes II

This activity contains 15 questions.

A "privacy leak" occurs:

If an instance variable is declared to be public or if an accessor method returns a reference to a private variableIf a local variable is declared to be public, or if an accessor method returns a reference to a private variableIf the value of a private instance variable is changed by a public mutator methodIf a static variable is declared to be public and final, or if a mutator method returns void

A copy constructor should:

be a constructor with a single argument of the same type as the class
create an entirely separate object, with identical data values as in the original object
never use the new method (because it causes data leaks)
be created when a class contains instance variables that are of non-primitive types

Only statement (d) is falseAll four statements are trueOnly statement (c) is false

How are static variables initialized?

Static variables are initialized in the same manner as instance variables, but their values are automatically "final"Static variables are initialized, and re-initialized, each time the class (or a method within the class) is usedStatic variables are automatically initialized to zero (or whatever zero means for the particular type)Static variables are left uninitialized, just like local variables

Most, but not all, of the wrapper classes are spelled the same as the keyword for the data type that they "wrap," but with an initial capital letter. Which wrappers don't follow this rule?

Integer, Character.Float, Double.Character, String.Integer, Short, Byte.

Packages provide:

a way to group a collection of classes into a library
a way to deal with name clashes
a way to override the default CLASSPATH
a way to integrate Javadoc-generated output with Javac-generated output

All four statements are trueStatements (a), (b), and (d) are trueStatements (a) and (b) are trueStatements (a), (b), and (c) are true

What is an immutable class?

An immutable class is one that has no mutator methodsAn immutable class is one having no accessor or mutator methodsAn immutable class is one that ONLY contains private instance variablesAn immutable class is one that has no methods (other than constructors) that change any of the instance data within the class

What is printed?

public class Simple {

	public static void main(String[] args) {
		String str = "-12";
		str += "34";
		float val = 
			Integer.parseInt(str + '5');
		System.out.println("val = " + val);
	}

}

val = -12345val = -125.0val = 12345.0val = -12345.0

What is printed?

public class Simple {

	public static void main(String[] args) {
		Integer num = new Integer(7 + 9);
		double dbl = num.doubleValue() / 2;
		System.out.println("dbl = " + dbl);
	}

}

dbl = 39.5dbl = 16.0dbl = 8.0dbl = 16

What is printed?

public class Simple {

	Simple() {
		value = (int) (value / 10);
	}

	public String toString() {
		return "" + value;
	}

	public static void main(String[] args) {
		Simple simple = null;
		for (int n = 0; n < 5; n++)
			simple = new Simple();
		System.out.println(simple);
	}

	private static double value = 12345678;
}

1234567.012.0123.0123

What is printed?

public class Simple {

	Simple() {
		value = 3;
	}

	void update(Simple s) {
		s.value *= s.value;
	}

	public String toString() {
		return "" + value;
	}

	public static void main(String[] args) {
		Simple s = new Simple();
		Simple t = new Simple();
		s.update(t);
		System.out.println(s + " " + t);
	}

	private short value;
}

9 33 93 39 9

What is the difference between a deep copy and a shallow copy?

A deep copy never has the same values as does a shallow copyA deep copy always uses a copy constructor to create a copy of primitive instance data, except possibly for immutable objectsA deep copy always uses a copy constructor to create a copy of non-primitive instance data, except possibly for immutable objectsA shallow copy always uses a copy constructor to create a copy of primitive instance data, except possibly for immutable objects

When is an object "anonymous?"

Objects cannot be anonymous -- they always have namesAn object is anonymous if you don't assign its reference to a variableAn object is anonymous if its reference is saved in a variable of a more general typeAn object is anonymous if it is created using the default constructor for the class

Which of these short programs correctly reports the number of Blobs that are created?

public class Blob {

	static int count = 0;

	Blob() { ++count; }

	public void report() {
		System.out.println(count+" Blobs");
	}

	public static void main(String[] args) {
		int LIMIT = (int)(20*Math.random());
		for (int n = 0; n < LIMIT; n++)
			new Blob();
		Blob.report();
	}

}

public class Blob {

	static int count = 0;

	Blob() { ++count; }

	static void report() {
		System.out.println(count+" Blobs");
	}

	public static void main(String[] args) {
		int LIMIT = (int)(20*Math.random());
		for (int n = 0; n < LIMIT; n++)
			new Blob();
		Blob.report();
	}

}

public class Blob {

	final int count = 0;

	Blob() { ++count; }

	static void report() {
		System.out.println(count+" Blobs");
	}

	public static void main(String[] args) {
		int LIMIT = (int)(20*Math.random());
		for (int n = 0; n < LIMIT; n++)
			new Blob();
		Blob.report();
	}

}

public class Blob {

	int count = 0;

	Blob() { ++count; }

	static void report() {
		System.out.println(count+" Blobs");
	}

	public static void main(String[] args) {
		int LIMIT = (int)(20*Math.random());
		for (int n = 0; n < LIMIT; n++)
			new Blob();
		Blob.report();
	}

}

Which of these short programs correctly reports the number of instances of Bleep that are created?

public class Blob {

	Blob() {
		int LIMIT = (int)(20*Math.random());
		Bleep bleep = null;
		for (int n = 0; n < LIMIT + 1; n++)
			bleep = new Bleep();
		bleep.report();
	}

	class Bleep {
		int count = 0;
		Bleep() { ++count; }
		public void report() {
			System.out.println(count +
					" Bleeps");
		}
	}

	public static void main(String[] args) {
		new Blob();
	}

}

public class Blob {

	int count = 0;

	Blob() {
		int LIMIT = (int)(20*Math.random());
		Bleep bleep = null;
		for (int n = 0; n < LIMIT + 1; n++)
			bleep = new Bleep();
		bleep.report();
	}

	class Bleep {
		Bleep() { ++count; }
		public void report() {
			System.out.println(count +
					" Bleeps");
		}
	}

	public static void main(String[] args) {
		new Blob();
	}

}

public class Blob {

	int count = 0;

	Blob() {
		int LIMIT = (int)(20*Math.random());
		Bleep bleep = null;
		for (int n = 0; n < LIMIT + 1; n++)
			new Bleep();
		Bleep.report();
	}

	class Bleep {
		Bleep() { ++count; }
		public void report() {
			System.out.println(count +
					" Bleeps");
		}
	}

	public static void main(String[] args) {
		new Blob();
	}
}

public class Blob {

	int count = 0;

	Blob() {
		int LIMIT = (int)(20*Math.random());
		Bleep bleep = null;
		for (int n = 0; n < LIMIT + 1; n++)
			new Bleep();
		Bleep.report();
	}

	class Bleep {
		Bleep() { ++count; }
		static void report() {
			System.out.println(count +
					" Bleeps");
		}
	}

	public static void main(String[] args) {
		new Blob();
	}

}

Which statement(s) is (are) true?

You can invoke a nonstatic method from within a static method
You cannot invoke a nonstatic method from within a static method
You can invoke a nonstatic method from within a nonstatic method
You can invoke a static method from within a static method

Only statements (c) and (d) are trueOnly statement (b) is falseStatements (b), (c), and (d) are trueAll four statements are true

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Chapter 5

A "privacy leak" occurs:

How are static variables initialized?

Most, but not all, of the wrapper classes are spelled the same as the keyword for the data type that they "wrap," but with an initial capital letter. Which wrappers don't follow this rule?

Packages provide:

a way to group a collection of classes into a library
a way to deal with name clashes
a way to override the default CLASSPATH
a way to integrate Javadoc-generated output with Javac-generated output

What is an immutable class?

What is printed?
public class Simple { public static void main(String[] args) { String str = "-12"; str += "34"; float val = Integer.parseInt(str + '5'); System.out.println("val = " + val); } }

What is printed?
public class Simple { public static void main(String[] args) { Integer num = new Integer(7 + 9); double dbl = num.doubleValue() / 2; System.out.println("dbl = " + dbl); } }

What is the difference between a deep copy and a shallow copy?

When is an object "anonymous?"

Which of these short programs correctly reports the number of Blobs that are created?

Which of these short programs correctly reports the number of instances of Bleep that are created?

Chapter 5

A "privacy leak" occurs:

How are static variables initialized?

Most, but not all, of the wrapper classes are spelled the same as the keyword for the data type that they "wrap," but with an initial capital letter. Which wrappers don't follow this rule?

Packages provide: a way to group a collection of classes into a library a way to deal with name clashes a way to override the default CLASSPATH a way to integrate Javadoc-generated output with Javac-generated output

What is an immutable class?

What is printed? public class Simple { public static void main(String[] args) { String str = "-12"; str += "34"; float val = Integer.parseInt(str + '5'); System.out.println("val = " + val); } }

What is printed? public class Simple { public static void main(String[] args) { Integer num = new Integer(7 + 9); double dbl = num.doubleValue() / 2; System.out.println("dbl = " + dbl); } }

What is the difference between a deep copy and a shallow copy?

When is an object "anonymous?"

Which of these short programs correctly reports the number of Blobs that are created?

Which of these short programs correctly reports the number of instances of Bleep that are created?

Packages provide:

a way to group a collection of classes into a library
a way to deal with name clashes
a way to override the default CLASSPATH
a way to integrate Javadoc-generated output with Javac-generated output

What is printed?
public class Simple { public static void main(String[] args) { String str = "-12"; str += "34"; float val = Integer.parseInt(str + '5'); System.out.println("val = " + val); } }

What is printed?
public class Simple { public static void main(String[] args) { Integer num = new Integer(7 + 9); double dbl = num.doubleValue() / 2; System.out.println("dbl = " + dbl); } }