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The `else` statement runs a block of code when the condition in the `if` statement is false. It is used as a catch-all when none of the previous conditions are true.
boolean isRaining = false;
if (isRaining) {
System.out.println("Take an umbrella.");
} else {
System.out.println("No need for an umbrella.");
}
// Prints: No need for an umbrella
`else if` statements are used to check multiple conditions one by one. As soon as one condition is true, its code block runs and the rest are skipped.
int temperature = 75;
if (temperature >= 90) {
System.out.println("It's very hot.");
} else if (temperature >= 70) {
System.out.println("It's warm.");
} else if (temperature >= 50) {
System.out.println("It's cool.");
} else {
System.out.println("It's cold.");
}
// Prints: It's warm.
An `if` statement checks a condition and runs code only if the condition is true. You can have multiple `else if` statements to check additional conditions.
if (5 > 3) {
System.out.println("5 is greater than 3.");
}
// Prints: 5 is greater than 3.
Nested conditionals are `if` statements inside other `if` statements. The outer `if` is checked first, and if it's true, the inner `if` is checked next.
boolean hasMoney = true;
boolean wantsToBuyIceCream = true;
if (hasMoney) {
System.out.println("You have money!");
if (wantsToBuyIceCream) {
System.out.println("You can buy ice cream!");
} else {
System.out.println("Maybe next time.");
}
}
// Prints: You have money!
// Prints: You can buy ice cream!
The `NOT` operator `!` inverts the value of a boolean expression. If the expression is true, `!` makes it false, and vice versa.
boolean isWeekend = true;
System.out.println(!isWeekend); // Prints: false
The `AND` operator `&&` checks if both conditions are true. If both are true, the result is true; otherwise, it's false.
boolean isSunny = true;
boolean isWarm = true;
System.out.println(isSunny && isWarm); // Prints: true
The `OR` operator `||` checks if at least one of the conditions is true. If at least one is true, the result is true.
boolean isWeekend = false;
boolean isHoliday = true;
System.out.println(isWeekend || isHoliday); // Prints: true
When using multiple logical operators, the order of evaluation is important: Parentheses first, then `NOT`, then `AND`, and finally `OR`.
boolean result = true && (!false || true); // true
/*
First, evaluate inside parentheses: (!false || true).
Then, !false becomes true, making it (true || true).
Finally, true && true results in true. */
You can compare object references using `==` to check if they point to the same object. For comparing values, use `equals()` method.
String a = new String("hello");
String b = new String("hello");
System.out.println(a == b); // Prints: false (different objects)
System.out.println(a.equals(b)); // Prints: true (same value)
Comparing primitive values (like numbers or characters) is straightforward using `==` or `!=` to check if they are equal or not.
int x = 10;
int y = 20;
System.out.println(x == y); // Prints: false
System.out.println(x != y); // Prints: true
Aliases refer to multiple references pointing to the same object. You can compare these references using `==`.
String a = new String("apple");
String b = new String("banana");
String c = b;
System.out.println(a == b); // Prints: false
System.out.println(b == c); // Prints: true
You can check if an object reference is `null` to avoid errors when trying to access its methods or properties.
String name = null;
if (name != null && name.contains("a")) {
System.out.println(name + " contains an 'a'.");
} else {
System.out.println("No 'a' found or name is null.");
}
// Prints: No 'a' found or name is null.
You can define your own `equals` method in a class to compare objects based on their attributes instead of their reference.
class Person {
String name;
int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null || getClass() != obj.getClass()) return false;
Person person = (Person) obj;
return age == person.age && name.equals(person.name);
}
}
Person p1 = new Person("Alice", 30);
Person p2 = new Person("Alice", 30);
System.out.println(p1.equals(p2)); // Prints: true
DeMorgan's Laws help simplify complex boolean expressions by transforming `AND` to `OR` and vice versa.
int x = 4;
int y = 5;
boolean result1 = !(x > y && x == y);
// Rewritten as: ! (x > y) || !(x == y)
boolean result2 = !(x < y || x != y);
// Rewritten as: ! (x < y) && !(x != y)
You can use DeMorgan's Laws to rewrite boolean expressions into simpler forms.
int a = 1;
int b = 2;
boolean exp1 = !(a == b && b >= a);
boolean exp2 = !(a == b) || !(b >= a);
boolean exp3 = a != b || a < b;
System.out.println(exp1); // Prints: true
System.out.println(exp2); // Prints: true
System.out.println(exp3); // Prints: true
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