Java If-Else Statements
Introduction
In programming, we often need our code to make decisions based on certain conditions. This is where conditional statements come into play, and the if-else statement is one of the most fundamental of these.
Java's if-else statements allow your program to execute different blocks of code depending on whether a specified condition evaluates to true or false. This ability to choose different execution paths is what makes programs dynamic and responsive to user input, data values, and other changing conditions.
Basic If Statement
The simplest form of conditional execution is the if statement. It evaluates a condition and executes a block of code only if that condition is true.
Syntax
if (condition) {
// code to execute if condition is true
}
Example
int temperature = 28;
if (temperature > 25) {
System.out.println("It's a hot day!");
}
// Output: It's a hot day!
In this example, since 28 is greater than 25, the message "It's a hot day!" is printed to the console.
If-Else Statement
What if we want to execute a different block of code when the condition is false? This is where the else clause comes in.
Syntax
if (condition) {
// code to execute if condition is true
} else {
// code to execute if condition is false
}
Example
int temperature = 20;
if (temperature > 25) {
System.out.println("It's a hot day!");
} else {
System.out.println("It's not a hot day.");
}
// Output: It's not a hot day.
In this case, since 20 is not greater than 25, the code inside the else block executes, printing "It's not a hot day."
If-Else-If Ladder
Often, we need to check multiple conditions in sequence. The if-else-if ladder (also called an if-else-if chain) allows us to test several conditions one after the other.
Syntax
if (condition1) {
// code to execute if condition1 is true
} else if (condition2) {
// code to execute if condition2 is true
} else if (condition3) {
// code to execute if condition3 is true
} else {
// code to execute if none of the conditions are true
}
Example
int temperature = 20;
if (temperature > 30) {
System.out.println("It's very hot!");
} else if (temperature > 25) {
System.out.println("It's hot!");
} else if (temperature > 15) {
System.out.println("It's warm.");
} else {
System.out.println("It's cold.");
}
// Output: It's warm.
The program checks each condition in order. Since temperature is 20, which is greater than 15, it prints "It's warm." Once a condition evaluates to true, the corresponding block executes, and the rest of the conditions in the ladder are skipped.
Nested If-Else
You can also place if-else statements inside other if-else blocks. This is called nesting, and it allows for more complex decision-making logic.
Syntax
if (outerCondition) {
// code to execute if outerCondition is true
if (innerCondition) {
// code to execute if both outerCondition and innerCondition are true
} else {
// code to execute if outerCondition is true but innerCondition is false
}
} else {
// code to execute if outerCondition is false
}
Example
int age = 25;
boolean hasLicense = true;
if (age >= 18) {
System.out.println("You are an adult.");
if (hasLicense) {
System.out.println("You can drive a car.");
} else {
System.out.println("You need to get a license to drive.");
}
} else {
System.out.println("You are a minor.");
}
/* Output:
You are an adult.
You can drive a car.
*/
This program first checks if the person is an adult. Since age is 25, which is greater than or equal to 18, it executes the first block. Inside that block, it checks if the person has a license. Since hasLicense is true, it also prints "You can drive a car."
Using Logical Operators in Conditions
You can use logical operators to combine multiple conditions:
&&(AND) - Both conditions must be true||(OR) - At least one condition must be true!(NOT) - Negates a condition
Example
int age = 25;
boolean hasLicense = true;
boolean hasInsurance = true;
if (age >= 18 && hasLicense && hasInsurance) {
System.out.println("You can legally drive your car.");
} else if (age >= 18 && hasLicense && !hasInsurance) {
System.out.println("You need insurance to legally drive.");
} else if (age >= 18 && !hasLicense) {
System.out.println("You need to get a license first.");
} else {
System.out.println("You are too young to drive.");
}
// Output: You can legally drive your car.
Flow of If-Else Statements
Let's visualize how the flow of execution works in if-else statements:
Common Pitfalls and Best Practices
1. Forgetting Curly Braces
If you omit curly braces, only the first statement following the condition is considered part of the if or else block.
// Risky code
if (temperature > 30)
System.out.println("It's very hot!");
System.out.println("Drink water!"); // This always executes regardless of the condition!
// Better code
if (temperature > 30) {
System.out.println("It's very hot!");
System.out.println("Drink water!");
}
2. Using == for String Comparison
For String comparisons, use the .equals() method instead of ==:
// Incorrect
String name = "John";
if (name == "John") { // Don't do this!
System.out.println("Hello, John!");
}
// Correct
if (name.equals("John")) {
System.out.println("Hello, John!");
}
3. Excessive Nesting
Too many nested if-else statements can make your code hard to read and maintain. Consider refactoring using early returns, switch statements, or separate methods.
Practical Examples
1. Simple Login System
String storedUsername = "user123";
String storedPassword = "pass456";
String inputUsername = "user123";
String inputPassword = "pass456";
if (inputUsername.equals(storedUsername)) {
if (inputPassword.equals(storedPassword)) {
System.out.println("Login successful!");
} else {
System.out.println("Incorrect password!");
}
} else {
System.out.println("Username not found!");
}
// Output: Login successful!
2. Grade Calculator
int score = 85;
char grade;
if (score >= 90) {
grade = 'A';
} else if (score >= 80) {
grade = 'B';
} else if (score >= 70) {
grade = 'C';
} else if (score >= 60) {
grade = 'D';
} else {
grade = 'F';
}
System.out.println("Your grade is: " + grade);
// Output: Your grade is: B
3. Discount Calculator for an Online Store
double totalPurchase = 120.0;
boolean isMember = true;
double discount = 0.0;
double finalPrice;
if (isMember) {
if (totalPurchase >= 100.0) {
discount = 0.15; // 15% discount for members purchasing $100+
} else {
discount = 0.10; // 10% discount for members purchasing less than $100
}
} else {
if (totalPurchase >= 100.0) {
discount = 0.05; // 5% discount for non-members purchasing $100+
}
}
finalPrice = totalPurchase * (1 - discount);
System.out.println("Total purchase: $" + totalPurchase);
System.out.println("Discount: " + (discount * 100) + "%");
System.out.println("Final price: $" + finalPrice);
/* Output:
Total purchase: $120.0
Discount: 15.0%
Final price: $102.0
*/
Summary
In this lesson, we've covered:
- The basic
ifstatement for executing code when a condition is true - The
if-elsestatement for handling both true and false conditions - The
if-else-ifladder for checking multiple conditions in sequence - Nested
if-elsestatements for more complex decision-making - Using logical operators in conditions to combine multiple checks
- Common pitfalls and best practices when using conditional statements
- Practical examples of conditional statements in real-world scenarios
Understanding if-else statements is crucial for controlling the flow of your Java programs. As you become more comfortable with these concepts, you'll be able to write increasingly sophisticated logic to handle a wide range of scenarios.
Exercises
- Write a program that checks if a number is positive, negative, or zero.
- Create a BMI calculator that outputs different messages based on the calculated BMI value.
- Implement a simple calculator that performs addition, subtraction, multiplication, or division based on user input.
- Write a program that determines if a year is a leap year.
- Create a program that recommends an activity based on weather conditions and temperature.
Additional Resources
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