Java 8 OCA Chapter 1

The difference between an object and a reference?

*Reference* A reference may or may not be created on the heap. All references are the same size, no matter what their data type is, and are accessed by their variable name. *Object* Objects are always on the heap. They have no name and can only be accessed via a reference. Objects vary in size depending on their class definition.

Specific number literals?

- In addition to "normal" numbers, numeric literals are allowed to begin with 0 (octal), 0x (hex), 0X (hex), 0b (binary), or 0B (binary). - Numeric literals are also allowed to contain underscores as long as they are directly between two other numbers. double notAtStart = _1000.00; // NOT COMPILE double notAtEnd = 1000.00_; // NOT COMPILE double notByDecimal = 1000_.00; // NOT COMPILE double annoyingButLegal = 1_00_0.0_0; // compiles

Which imports are compilable? 1. import java.util.*; import java.sql.*; 2. import java.util.Date; import java.sql.*; 3. import java.util.Date; import java.sql.Date;

1. No because both packages have Date class. 2. Yes because if you explicitly import a class name, it takes precedence over any wildcards present. 3. No

What is the output of the following code? 1: public class Deer { 2: public Deer() { System.out.print("Deer"); } 3: public Deer(int age) { System.out.print("DeerAge"); } 4: private boolean hasHorns() { return false; } 5: public static void main(String[] args) { 6: Deer deer = new Reindeer(5); 7: System.out.println(","+deer.hasHorns()); } 8: } 9:} 10: class Reindeer extends Deer { 11: public Reindeer(int age) { System.out.print("Reindeer"); } 12: public boolean hasHorns() { return true; } 13: } A. DeerReindeer,false B. DeerReindeer,true C. ReindeerDeer,false D. ReindeerDeer,true E. DeerAgeReindeer,false F. DeerAgeReindeer,true G. The code will not compile because of line 7. H. The code will not compile because of line 12.

A. The code compiles and runs without issue, so options G and H are incorrect. First, the Reindeer object is instantiated using the constructor that takes an int value. Since there is no explicit call to the parent constructor, the default no-argument super() is inserted as the first line of the constructor. The output is then Deer, followed by Reindeer in the child constructor, so only options A and B can be correct. Next, the method hasHorns() looks like an overridden method, but it is actually a hidden method since it is declared private in the parent class. Because the hidden method is referenced in the parent class, the parent version is used, so the code outputs false, and option A is the correct answer.

What is the output of the following program? 1: public class FeedingSchedule { 2: public static void main(String[] args) { 3: boolean keepGoing = true; 4: int count = 0; 5: int x = 3; 6: while(count++ < 3) { 7: int y = (1 + 2 * count) % 3; 8: switch(y) { 9: default: 10: case 0: x -= 1; break; 11: case 1: x += 5; 12: } 13: } 14: System.out.println(x); 15: }} 15: A. 4 B. 5 C. 6 D. 7 E. 13 F. The code will not compile because of line 7

C

What is the output of the following code? (Choose all that apply) 1: interface HasTail { int getTailLength(); } 2: abstract class Puma implements HasTail { 3: protected int getTailLength() {return 4;} 4: } 5: public class Cougar extends Puma { 6: public static void main(String[] args) { 7: Puma puma = new Puma(); 8: System.out.println(puma.getTailLength()); 9: } 10: 11: public int getTailLength(int length) {return 2;} 12: } A. 2 B. 4 C. The code will not compile because of line 3. D. The code will not compile because of line 5. E. The code will not compile because of line 7. F. The code will not compile because of line 11. G. The output cannot be determined from the code provided.

C, D, E C: protected is a more restrictive access modifier than public D: the class Cougar implements an overloaded version of getTailLength(), but since the declaration in the parent class Puma is invalid, it needs to implement a public version of the method. Since it does not, the declaration of Puma is invalid E: Puma is marked abstract

Which are true of the following code? (Choose all that apply) 1: import java.util.*; 2: public class Grasshopper { 3: public Grasshopper(String n) { 4: name = n; 5: } 6: public static void main(String[] args) { 7: Grasshopper one = new Grasshopper("g1"); 8: Grasshopper two = new Grasshopper("g2"); 9: one = two; 10: two = null; 11: one = null; 12: } 13: private String name; } A: Immediately after line 9, no grasshopper objects are eligible for garbage collection. B: Immediately after line 10, no grasshopper objects are eligible for garbage collection. C: Immediately after line 9, only one grasshopper object is eligible for garbage collection. D: Immediately after line 10, only one grasshopper object is eligible for garbage collection. E: Immediately after line 11, only one grasshopper object is eligible for garbage collection. F: The code compiles. G: The code does not compile.

C, D, F. Immediately after line 9, only Grasshopper g1 is eligible for garbage collection since both one and two point to Grasshopper g2. Immediately after line 10, we still only have Grasshopper g1 eligible for garbage collection. Reference one points to g1 and reference two is null. Immediately after line 11, both Grasshopper objects are eli- gible for garbage collection since both one and two point to null. The code does com- pile. Although it is traditional to declare instance variables early in the class, you don't have to.

Are there any class file name restrictions?

Class file name must match the name of public class if the file contains one (one class file is allowed to contain one public class most and many

Will this code compile? public void findAnswer(boolean check) { int answer; int onlyOneBranch; if (check) { onlyOneBranch = 1; answer = 1; } else { answer = 2; } System.out.println(answer); // 1 System.out.println(onlyOneBranch); // 2 }

Compiler will complain about statement 2 only.

Given the following class in the file /my/directory/named/A/Bird.java: INSERT CODE HERE public class Bird { } Which of the following replaces INSERT CODE HERE if we compile from/my/directory? (Choose all that apply) A.package my.directory.named.a; B.package my.directory.named.A; C.package named.a; D.package named.A; E.package a; F.package A; G. Does not compile.

D, package names are case sensitive

How declaring a variable differs from initializing it?

Declaring: String s1; Initializing: s1 = "Some string value"; Declaring and initializing in one short: String s1 = "Some strong value"

Order of instance Initialization?

Fields and instance initializer blocks are run in the order in which they appear in the file. The constructor runs after all fields and instance initializer blocks have run.

How import and package statements are applied to classes within the same source code file?

Import and package statements apply to all classes within a source code file. In other words, there's no way to declare multiple classes in a file and have them in different packages, or use different imports

How constructor differs from other methods?

It has the same name with class and ommites return type

Will it compile? long max = 3123456789;

No. When a number is present in the code, it is called a literal. By default, Java assumes you are defining an int value with a literal. In this example, the number listed is bigger than what fits in an int. It would compile: long max = 3123456789L;

How does null assignment works for primitives and reference types? Is there a difference?

Reference types can be assigned null, which means they do not currently refer to an object. Primitive types will give you a compiler error if you attempt to assign them null. int value = null; // DOES NOT COMPILE String s = null;

What are rules for legal identifiers?

The name must begin with a letter or the symbol $ or _. Subsequent characters may also be numbers. You cannot use the same name as a Java reserved word.

Enlist reserved words (53)

abstract assert boolean break byte case catch char class const* continue default do double else enum extends false final finally float for goto* if implements import instanceof int interface long native new null package private protected public return short static strictfp super switch synchronized this throw throws transient true try void volatile while

Is there a garantee to have finalize() method called?

finalize() is called either zero or once (even if finalize() method fails or reanimates the object by adding its reference to scope)

How to execute java class from command line?

javac org/exam/MtClass.java java org.exam.MyClass "arg1" "arg2" java org.exam.MyClass "arg1" arg2 java -cp ".;C:\temp\someOtherLocation;c:\temp\myJar.jar" org.exam.MyClass Executing main class is NOT allowed with class defined like MyClass.class. Only name without extension is allowed

Executable main() method signature options?

public static void main(String[] args) public static void main(String args[]) public static void main(String... args) public static void main(String[] args) throws Exception

What are the rules for multiple variable declaring?

variable declaring? 1. Variables are all of the same type 2. Type is only mentioned once: 3. You can also initialize any or all of those values inline. boolean b1, b2; // ok String s1 = "1", s2; // ok double d1, double d2; // DOES NOT COMPILE int i1; int i2; // ok int i3; i4; // DOES NOT COMPILE

3 types of variables (based on the scope)?

■ Class variable ■ Instance variable ■ Local variable

Benefits of Java?

■ Object oriented ■ Encapsulation ■ Platform independent ■ Robust ■ Simple ■ Secure

When an object is eligible for garbage collection?

■ The object no longer has any references pointing to it. ■ All references to the object have gone out of scope.