Obj 3: Utilizing Java Object-Oriented Approach

Question 1

what is an object ?

Answer 1

An object is a runtime instance of a class
in memory.

Question 2

what is a reference ?

Answer 2

A reference is a variable that points to an object

Question 3

what are the members of a class ?

Answer 3

Java classes have two primary elements: methods (often called functions or procedures) and fields (known as variables

Variables hold the state of the program, and methods operate on that state. If the change is important to remember, a variable stores that change

Question 4

what is a method signature ?

Answer 4

The method name and parameter types are called the method signature

Question 5

what does the main() method ?

Answer 5

The main() method lets the JVM call our code. The simplest possible class with a main() method looks like this:
public class Zoo {
public static void main(String[] args) {
System.out.println(“Hello World”);
} }

Question 6

what are packages ?

Answer 6

Java puts classes in packages. These are logical groupings for classes.

Question 7

import java.util.*
does it import child packages, fields or methods ?

Answer 7

The * is a wildcard that matches all classes in the package
The import statement doesn’t bring in
child packages, fields, or methods; it imports only classes directly under the package

Question 8

import java.util.Date;
 import java.sql.Date;

does it compile ?

Answer 8

No, Because there can’t be two defaults, the compiler tells you the imports are
ambiguous.

do this instead
~~~
public class Conflicts {
java.util.Date date;
java.sql.Date sqlDate;
}
~~~

Question 8

what is reference type ?

Answer 8

A reference type refers to an object (an instance of a class). Unlike primitive types that hold their values in the memory where the variable is allocated, references do not hold the value of the object they refer to. Instead, a reference “points” to an object by storing the memory address where the object is located, a concept referred to as a pointer.

Question 9

Park p = new Park();

Answer 9

First you declare the type that you’ll be creating (Park) and give the variable a name (p)

This gives Java a place to store a reference to the object. Then you write new Park() to
actually create the object.

Question 9

public class Chick {
   public Chick() {
      System.out.println("in constructor");
   }

 public class Chick {
   public void Chick() { }  // NOT A CONSTRUCTOR
 }
 }

Answer 9

There are two key points to note about the constructor: the name of the constructor
matches the name of the class, and there’s no return type.

The purpose of a constructor is to initialize fields, although you can put any code in there.

Question 10

String str = new String("hello !");
String str_ = str;

does it compile ?

Answer 10

A value is assigned to a reference in one of two ways:
- A reference can be assigned to another object of the same or compatible type.
- A reference can be assigned to a new object using the new keyword.

Question 11

what is the order of initialization ?

Answer 11

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

Question 12

which one is decalaring a variable or initializing a variable
~~~
String str;
String str_ = “hello!”;
~~~

Answer 12

A variable is a name for a piece of memory that stores data. When you declare a variable, you need to state the variable type along with giving it a name. Giving a variable a value is called initializing a variable. To initialize a variable, you just type the variable name followed by an equal sign, followed by the desired value.

Question 13

What is local variable ?

Answer 13

A local variable is a variable defined within a constructor, method, or initializer block

Question 14

final int y = 10;
 int x = 20;
 y = x + 10;

does it compile ?

Answer 14

No, The final keyword can be applied to local variables and is equivalent to declaring constants in other languages

Question 15

final int[] favoriteNumbers = new int[10];
 favoriteNumbers[0] = 10;
favoriteNumbers[1] = 20;
 favoriteNumbers = null; 

does it compile ?

Answer 15

No, we can modify the content, or data, in the array. The code does not compile when we try to change the value of the reference

Question 16

public int do() {
   int y = 10;
    int x;
    int reply = x + y;  
    return reply;
 }

Does it compile ?

Answer 16

No, Local variables do not have a default value and must be initialized before use.
the compiler will report an error if you try to read an uninitialized value

Question 17

public void findAnswer(boolean check) {
   int answer;
   int otherAnswer;
   int onlyOneBranch;
   if (check) {
      onlyOneBranch = 1;
      answer = 1;
  } else {
     answer = 2;
  }
   System.out.println(answer);
   System.out.println(onlyOneBranch); 
	 }

Does it compile ?

Answer 17

No, The onlyOneBranch variable is initialized only if check happens to be true. The compiler knows there is the possibility for check to be false, resulting in uninitialized code, and gives a compiler error.

On the exam, be wary of any local variable that is declared but not initialized in a single line. This is a common place on the exam that could result in a “Does not compile” answer. Be sure to check to make sure it’s initialized before it’s used on the exam.

Question 18

public class Car {
   private static String color;
   private String brand;
   public static void main(String[] args) {
     System.out.println(color);
     System.out.println(brand);
}

which one is the class variable and the instance variable ?

does it compile ?

Answer 18

no, compiler throws error “non-static variable brand cannot be referenced from a static context”
you should initialize instance variables
color -> class variable
brand -> instance variable

An instance variable, often called a field, is a value defined within a specific instance of
an object

a class variable is one that is defined on the class level and shared among all instances of the class. It can even be publicly accessible to classes outside the class and doesn’t require an instance to use

Question 19

public class VarKeyword {
   var tricky = "Hello"; 
 }

Does it compile ?

Answer 19

No, Local variable type inference works with local variables and not instance variables

Question 20

public void reassignment() {  
    var number = 7;
    number = 4;
    number = "five";  
 }

Does it compile ?

Answer 20

No, When you type var, you are instructing the compiler to determine the type for you. The compiler looks at the code on the line of the declaration and uses it to infer the type.

In Java, var is still a specific type defined at compile time. It does not change type at runtime.

Question 21

public void doesThisCompile(boolean check) {
      var question;
      question = 1;
      var answer;
      if (check) {
        answer = 2;
     } else {
       answer = 3;
     }
    System.out.println(answer);
 }

does this compile ?

Answer 21

No, Since question and answer are not assigned values on the lines where they are defined, the compiler does not know what to make of them

Question 22

~~~
public void twoTypes() {
int a, var b = 3;
var n = null;
}
```
Does this compile ?

Answer 22

No, All the types declared on a single line must be the same type and share the same declaration.

The compiler is being asked to infer the type of null. This could be any reference type. The only choice the compiler could make is Object. However, that is almost certainly not what the author of the code intended. The designers of Java decided it
would be better not to allow var for null than to have to guess at intent.

Question 23

var n = "Hello!";
n = null;

does this compile ?

Answer 23

yes, While a var cannot be initialized with a null value without a type, it can
be reassigned a null value after it is declared, provided that the under
lying data type is a reference type.

Question 24

public int addition(var a, var b) {  
   return a + b;
 }

does this compile ?

Answer 24

No, a and b are method parameters. These are not local variables. Be on the lookout for var used with constructors, method parameters, or instance variables. Using var in one of these places is a good exam trick to see if you are paying attention. Remember that var is only used for local variable type inference

Question 25

package var;
 public class Var {
   public void var() {
      var var = "var";
   }
   public void Var() {
      Var var = new Var();
   }
 }

does this compile ?

Answer 25

yes
var is not a reserved word and allowed to
be used as an identifier

this code does compile. Java is case sensitive, so Var doesn’t introduce
any conflicts as a class name. Naming a local variable var is legal.

Question 26

public void eatIfHungry(boolean hungry) {
   if (hungry) {
        int bitesOfCheese = 1;
   } 
    System.out.println(bitesOfCheese);  
 }

does it compile ?

Answer 26

No, The variable hungry has a scope of the entire method, while the variable bitesOfCheese
has a smaller scope. It is only available for use in the if statement because it is declared inside
of it. When you see a set of braces ({}) in the code, it means you have entered a new block of
code. Each block of code has its own scope

Local variables can never have a scope larger than the method they are defined in. However,
they can have a smaller scope.

Question 27

 public class Mouse {
     final static int MAX_LENGTH = 5;
     int length;
     public void grow(int inches) {
        if (length < MAX_LENGTH) {
           int newSize = length + inches;
           length = newSize;
       }
     }
 }

does this compile ?

Answer 27

Yes, instance variables are available as soon as they are defined and last for the entire lifetime of the object itself. The class, aka static, variables go into scope when declared like the other variable types. However, they stay in scope for the entire life of the program.

Local variables: In scope from declaration to the end of the block
Method parameters: In scope for the duration of the method
Instance variables: In scope from declaration until the object is eligible for garbage collection
Class variables: In scope from declaration until the program ends

Question 28

what is garbage collection ?

Answer 28

Garbage collection refers to the process of automatically freeing memory on the heap by
deleting objects that are no longer reachable in your program.

Garbage collection is responsible for removing
objects from memory when they can never be used again. An object becomes eligible for
garbage collection when there are no more references to it or its references have all gone
out of scope.

Question 29

What is the built-in method that runs the garbage colection where you can suggest ? is it guranteed to do anything ?

Answer 29

System.gc();
Java is free to ignore you. This method is not guaranteed to
do anything.

Question 30

When an object is no longer reachable ?

Answer 30

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

Question 31

Objects vs References

Answer 31

Do not confuse a reference with the object that it refers to; they are two different entities.
The reference is a variable that has a name and can be used to access the contents of an
object. A reference can be assigned to another reference, passed to a method, or returned
from a method. All references are the same size, no matter what their type is.

An object sits on the heap and does not have a name. Therefore, you have no way to access
an object except through a reference. Objects come in all different shapes and sizes and
consume varying amounts of memory. An object cannot be assigned to another object, and
an object cannot be passed to a method or returned from a method. It is the object that gets
garbage collected, not its reference

When you are asked a question about garbage collection on the exam, we recommend that you draw what’s going on.

Question 32

public void openZoo(Number time) {
   if(time instanceof String) 
      System.out.print(time);
 }

Does this compile ?

Answer 32

No, Number cannot possibly hold a String value, so the following causes a compilation error.
If the compiler can determine that a variable cannot possibly be cast to a specific class, it reports an error.

Question 33

 System.out.print(null instanceof Object);  // false
 Object noObjectHere = null;
 System.out.print(noObjectHere instanceof String);  // false

 System.out.print(null instanceof null);  // DOES NOT COMPILE

Answer 33

you should know that calling instanceof on the null literal or a null reference always returns false

This example does not compile, since null is used on the right side of the instanceof operator