Java Boolean Expressions — true, false & Conditions

Every decision in a program boils down to a boolean expression — something that evaluates to true or false. Before we learn if-else, while, or for, we need to master booleans.

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The boolean Type

boolean isActive = true;
boolean isDeleted = false;

System.out.println(isActive);   // true
System.out.println(isDeleted);  // false

A boolean can only hold two values: true or false. No 0 or 1, no "yes" or "no" — just true and false.

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Comparison Operators Produce Booleans

int a = 10, b = 20;

System.out.println(a == b);   // false
System.out.println(a != b);   // true
System.out.println(a < b);    // true
System.out.println(a > b);    // false
System.out.println(a <= 10);  // true
System.out.println(a >= 20);  // false

Every comparison expression returns a boolean.

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Combining Booleans with Logical Operators

AND (&&) — both must be true

int age = 25;
boolean hasLicense = true;

boolean canDrive = (age >= 18) && hasLicense;  // true

OR (||) — at least one must be true

boolean isWeekend = true;
boolean isHoliday = false;

boolean dayOff = isWeekend || isHoliday;  // true

NOT (!) — inverts the value

boolean isLoggedIn = false;
System.out.println(!isLoggedIn);  // true

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Truth Tables

AND (&&)

A	B	A && B
true	true	true
true	false	false
false	true	false
false	false	false

OR (||)

A	B	A || B
true	true	true
true	false	true
false	true	true
false	false	false

NOT (!)

A	!A
true	false
false	true

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Short-Circuit Evaluation

Java stops evaluating as soon as the result is determined:

// && — if left is false, right is NEVER evaluated
if (false && someExpensiveMethod()) {
    // someExpensiveMethod() never runs
}

// || — if left is true, right is NEVER evaluated
if (true || someExpensiveMethod()) {
    // someExpensiveMethod() never runs
}

Practical Use: Null Safety

String name = null;

// Safe — if name is null, length() is never called
if (name != null && name.length() > 0) {
    System.out.println(name);
}

Without short-circuit evaluation, name.length() would throw a NullPointerException.

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Complex Boolean Expressions

You can chain multiple conditions:

int age = 25;
double income = 50000;
boolean hasGoodCredit = true;

boolean loanApproved = (age >= 21) && (income >= 30000) && hasGoodCredit;

Use parentheses to make intent clear:

// Confusing
boolean result = a > 5 && b < 10 || c == 3;

// Clear
boolean result = (a > 5 && b < 10) || (c == 3);

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Boolean Methods

Methods can return booleans — by convention, they start with is, has, or can:

public static boolean isEven(int n) {
    return n % 2 == 0;
}

public static boolean isInRange(int n, int min, int max) {
    return n >= min && n <= max;
}

// Usage
if (isEven(42)) {
    System.out.println("Even!");
}

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Comparing Strings

Never use == for string comparison:

String a = new String("hello");
String b = new String("hello");

System.out.println(a == b);       // false ← compares references
System.out.println(a.equals(b));  // true  ← compares content

For case-insensitive comparison:

"Hello".equalsIgnoreCase("hello");  // true

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Boolean Pitfalls

1. Using = instead of ==

// WRONG — this is assignment, not comparison
if (x = 5) { }  // compile error (Java catches this!)

// CORRECT
if (x == 5) { }

2. Redundant boolean comparisons

// Bad
if (isActive == true) { }
if (isDeleted == false) { }

// Good
if (isActive) { }
if (!isDeleted) { }

3. De Morgan’s Laws

These are useful for simplifying expressions:

!(A && B)  is the same as  !A || !B
!(A || B)  is the same as  !A && !B

// These are equivalent:
if (!(age >= 18 && hasID)) { }
if (age < 18 || !hasID) { }

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Quick Exercises

1. Write a boolean expression that’s true if a number is between 1 and 100 (inclusive).
2. Write a method isLeapYear(int year) that returns true for leap years.
3. Simplify: !(x > 5 && y < 10) using De Morgan’s Law.

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Summary

• boolean holds true or false
• Comparison operators (==, !=, <, >, <=, >=) return booleans
• Logical operators (&&, ||, !) combine booleans
• Short-circuit evaluation skips unnecessary work
• Use .equals() for string comparison, not ==
• Name boolean methods with is, has, can prefixes

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What’s Next?

Now that you understand boolean expressions, you’re ready for decision-making — the if-else statement: If-Else in Java.