Java Basics

Question 1

Reference types

Answer 1

Reference types refers to objects that they refer to.

A reference points to an object by storing the memory address where teh object is located

Question 2

Can I assign a String refence variable to Integer later?

Answer 2

No, you cannot assign reference variable to different type.

A reference can be assigned to another object of same type.

Question 3

What is important about local variables?

Answer 3

You cannot use local variables without initializing.

It cannot be assigned to default value. It contains garbage value if not assigned any value.

If you try to use it before assigning, compile time error occurs

Question 4

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

Will this compile?

Answer 4

No, it will not compile.

The compiler throws error.

It complains saying that the the field- onlyOneBranch might have not been initialized

Question 5

Which method when overriden, called by Garbage collector?

Answer 5

finalize()

Question 6

How many times a finally could be called by GC?

Answer 6

Zero or one

Question 7

What are the rules for automatic numeric promotion?

Answer 7

If two values have different data types, Java will automatically promote one of the values
to the larger of the two data types.
2. If one of the values is integral and the other is floating-point, Java will automatically
promote the integral value to the floating-point value’s data type.

3. Smaller data types, namely byte, short, and char, are first promoted to int any time
   they’re used with a Java binary arithmetic operator, even if neither of the operands is
   int.
4. After all promotion has occurred and the operands have the same data type, the resulting
   value will have the same data type as its promoted operands

Question 8

int x = 3;
int y = ++x * 5 / x– + –x;
System.out.println(“x is “ + x);
System.out.println(“y is “ + y);

Answer 8

X=2

y=7

Question 9

short x = 10;
short y = 3;
short z = x * y;

What is the actual result?

Answer 9

Compilation error.

We have to downcast it like
short z=(short) (x*y)

Question 10

What is special about compound operator?

Answer 10

Downcasting will be automatically taken care by compound operator

Question 11

5 == 5.00 -> result?

Answer 11

TRUE

Question 12

int a=10;
int b = (a=2);
Syso(a );
syso(b); output?

Answer 12

a=2, b=2

Question 13

int y = 1;
int z = 1;
final int x = y<10 ? y++ : z++;
System.out.println(y+”,”+z); // Output?

Answer 13

2, 1

Question 14

What should we keep in mind regarding Switch statements?

Answer 14

The value in each case statement must be a compile time contsant. Means it should be final modifier with constant value assigned to it

Question 15

Output?

int x = 0;
for(long y = 0, x = 4; x < 5 && y < 10; x++, y++) {
System.out.print(x + “ “);
}

Answer 15

Compilation error- cannot redeclare the variable in for loop initialix=zation block

Question 16

String x=”Hello”;
String y =”Hello”.trim()

x==y?

Answer 16

false.

Because, one is computed at runtime

Question 17

three ways of creating an array

Answer 17

1. int[] abc=new int[3];
2. int[] abc=new int[] {1,2,3};
3. int[] abc={1,2,3}

Question 18

what is the method that prints the elements of the array nicely?

Answer 18

java.util.Arrays.toString()

Question 19

String[] strings = { “stringValue” };

4: Object[] objects = strings;
5: String[] againStrings = (String[]) objects;
6: againStrings[0] = new StringBuilder();
7: objects[0] = new StringBuilder();

Answer 19

line 6 throws compile time error, because StringBuilder cannot fit into String

line 7 throws runime error (ArrayStrore Exception)

Question 20

Method to sort elements in an array?

Answer 20

Arrays.sort()

Question 21

What is the requirement of using BinarySearch in array and what if it doesnt met?

Answer 21

The Array should have been sorted before using BinarySearch.. Otherwise, the result will be unpredictable

Question 22

How to convert to array from List?

Answer 22

list.toArray()

Question 23

what does list.toArray() defaults to?

Answer 23

Array of object

Object[] array = list.toArray();

Question 24

What do we need to do if we want String array from list ?

Answer 24

String[] array = list.toArray(new String[0])

Question 25

What is special about converting from Array to list?

Answer 25

The original array and backed array list are linked

Question 26

Creating List from Array?

Answer 26

Arrays.asList(array)

Question 27

private final ArrayList list=new ArrayList<>();

list.add(“String”);

Will it compile? If no, why? if Yes, why?

Answer 27

Yes, becasue we can change the state of the object here. Only problem is when we try to assign list to some other object like list=new ArrayList<>(); Now it throws compile time error

Question 28

private static int one;

15: private static final int two;
16: private static final int three = 3;
17: private static final int four;
18: static {
19: one = 1;
20: two = 2;
21: three = 3;
22: two = 4;
23: }

Which of these wont compile?

Answer 28

line 17- > because it is not initialized at 17 or in Static Initialization block

line 21 -> final vaiables cannot be reassignes
line 22 -> final variables cannot be reassigned

Question 29

static imports are meant only for what?

Answer 29

Static imports are meant only for importing static members.
You cant say
import static java.util.Arrays;

instead you can say
import static.java.util.Arrays.*;

Question 30

What are not a valid overloading?

Answer 30

1. Just changing the return type of the method and leaving others unchanged is not the valid overloading
2. Instance and static methods with the same method signatures

Question 31

public void fly(int[] lengths) { }
public void fly(int… lengths) { }

which method is called if we pass an int[]?

Answer 31

Compiler error.

Even though overloading is valid here.

Question 32

what is the difference between calling a method with VarArg parameters and array parameters?

for Eg..,

1. public void fly(int[] lengths) { }
2. public void fly(int… lengths) { }

what are the options to call these two methods?

Answer 32

Both can be called with array.

eg. .
1. fly(new int[]{1,2,3})

But, only varargs can eb called like below.
fly(1,2,3)

Question 33

which will be called when using fly(3);

public void fly(int numMiles) { }
public void fly(Integer numMiles) { }

Answer 33

method with primitive argument.

Java will call the method with most specific value type in the argument

Question 34

public class Glider2 {
public static String glide(String s) {
return "1";
}
public static String glide(String... s) {
return "2";
}
public static String glide(Object o) {
return "3";
}
public static String glide(String s, String t) {
return "4";
}
public static void main(String[] args) {
System.out.print(glide("a"));
System.out.print(glide("a", "b"));
System.out.print(glide("a", "b", "c"));
} }

Answer 34

142

Question 35

public class TooManyConversions {
public static void play(Long l) { }
public static void play(Long... l) { }
public static void main(String[] args) {
play(4); //1
play(4L); //2
} }

what happens on 1 and 2?

Answer 35

1. If an int is there, java is happy to convert it to long. means
   int -> long is fine in calling methods.
   but int -> long -> Long => Java will not do, so compilere error as we dont have methods that take int/long/object arguments
2. It calls the Long version of method

Question 36

what is the keyword to call constructer from another constructor in the same class?
And what is more important in that?

Answer 36

this

eg… this (a,b)

And this call must be the first statement inside the costructor from where we call

Question 37

When should be a final variable must have been assigned?

Answer 37

By the time constructor completes, all final variables must have been assigned

Question 38

what is functonal programming?

Answer 38

Functional progrmming is a way of writing the code more declaratively

Question 39

what are the variables that lambda can access?

Answer 39

1. Static
2. Instance
3. Method and local variables - provided there values are not changed

Question 40

(a, b) -> { int a = 0; return 5;} // will this compile?

Answer 40

No, lambda cannot change the parameters value passed

Question 41

What is the functional interface that hs a method that returns boolean?

Answer 41

Predicate.

public interface Predicate{
public boolean test(T t);
}

Question 42

what is the arrayList method that takes the Predicate?

Answer 42

removeIf()