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What are the differences between primitive types and reference types?
Primitive types store values directly; reference types store memory addresses. Primitive types start with lowercase; reference types start with uppercase.
What is the difference between `==` and `.equals()` when comparing objects?
`==` compares memory addresses (references); `.equals()` compares the content of the objects (if properly implemented).
Compare the use of the `String` class with the `Math` class.
String is used to create and manipulate text; Math provides static methods for mathematical operations. String objects are created with `new` or string literals; Math cannot be instantiated.
What is the difference between a static method and an instance method?
A static method belongs to the class itself and is called using the class name; an instance method belongs to an object of the class and is called using the object's name.
Compare Abstraction and Encapsulation.
Abstraction focuses on hiding complexity and showing only essential features, while encapsulation focuses on bundling data and methods that operate on that data, protecting it from outside access.
Compare Inheritance and Polymorphism.
Inheritance allows a class to inherit properties and behaviors from another class, promoting code reuse, while polymorphism allows objects of different classes to be treated as objects of a common type, enabling flexibility and extensibility.
What does the following code output?
```java
String str = "Hello";
String str2 = str;
str2 = "World";
System.out.println(str);
```
Hello
Identify the error in the following code:
```java
public class Example {
int x;
public static void main(String[] args) {
Example obj = new Example;
System.out.println(obj.x);
}
}
```
Missing parentheses in the constructor call: `new Example()`.
What does the following code output?
```java
public class Test {
int num;
public Test(int num) {
this.num = num;
}
public static void main(String[] args) {
Test t1 = new Test(10);
Test t2 = t1;
t2.num = 20;
System.out.println(t1.num);
}
}
```
20
What does the following code output?
```java
String str1 = new String("example");
String str2 = new String("example");
System.out.println(str1 == str2);
```
false
Identify the error in the following code:
```java
public class MyClass {
private int x;
public void MyClass(int x) { // Intended to be a constructor
this.x = x;
}
public static void main(String[] args) {
MyClass obj = new MyClass(5);
System.out.println(obj.x);
}
}
```
The 'constructor' `MyClass(int x)` is actually a method because it has a return type (void is implied). This means the value of x is never properly initialized, and x will default to 0, leading to a compilation error as the constructor is never called.
What does the following code output?
```java
public class Example {
public static void main(String[] args) {
String s1 = "Hello";
String s2 = "Hello";
System.out.println(s1 == s2);
}
}
```
true
What will be the output of this code?
```java
public class Example {
public static void main(String[] args) {
String str = "Coding";
System.out.println(str.substring(2, 4));
}
}
```
di
What does the following code output?
```java
public class Test {
int x = 5;
public static void main(String[] args) {
Test t1 = new Test();
Test t2 = new Test();
t1.x = 10;
System.out.println(t2.x);
}
}
```
5
What does the following code output?
```java
public class Example {
public static void main(String[] args) {
String s = "Hello World";
System.out.println(s.indexOf("World"));
}
}
```
6
What is wrong with the following code?
```java
public class Example {
public static void main(String[] args) {
Math m = new Math();
System.out.println(m.sqrt(25));
}
}
```
You cannot instantiate the Math class. All methods in the Math class are static, so you should call them using `Math.sqrt(25)`.
What is the role of a class in object creation?
A class serves as a blueprint, defining the structure and behavior of objects.
What does the `new` keyword do?
It's used to create a new object (instance) of a class, calling the constructor.
Explain the difference between a class and an object.
A class is a blueprint, while an object is a specific instance of that blueprint.
What is the significance of objects being reference types?
Objects store memory locations rather than actual values, affecting how they are handled in memory and during assignment.
What is the purpose of a constructor?
To initialize the state of an object when it is created.
What are the key principles of OOP?
Abstraction, Encapsulation, Inheritance, and Polymorphism.
What is the role of methods in a class?
Methods define the behaviors or actions that an object can perform.
Why is understanding objects important in Java?
Because Java is an object-oriented language, and objects are fundamental building blocks.
What does it mean for an object to be an instance of a class?
It means the object is a specific realization or example created from the class's blueprint.
What happens when you assign one object to another?
You are copying the reference (memory address), not creating a new, independent object.