Java if, else if, else Statements

Use the if-statement to test a variable. Apply the if-statement, else-if and else.

If, else. A frog's life is relatively simple. It decides where to sit, whether to croak, what to eat. Its decisions change future events.

In Java we navigate many more choices than the frog's. We use an if-statement (with optional "elses") to make decisions. We use expressions and operators.

First example. This program uses the if-statement in a loop. It also includes else-if and else blocks. The for-loop iterates through the values 0, 1 and finally 2.

Else: The else-statement here catches all cases not yet matched. The value 2, unmatched, ends up in the else-statement.

Tip: We call the System.out.println method, with a String argument, to display results to the console.

Console

Comparison: To compare a number, we use 2 equals signs. This is an expression. It evaluates to a true or false result.

Java program that uses if-statement public class Program { public static void main(String[] args) { // Loop through three numbers. for (int i = 0; i <= 2; i++) { if (i == 0) { System.out.println("Zero"); } else if (i == 1) { System.out.println("One"); } else { System.out.println("Else reached"); } } } } Output Zero One Else reached

Negation. To compare two numbers for inequality, we use the "!=" operator. We can also negate an entire expression by using a leading exclamation mark and parentheses.

Tip: Sometimes it is easier to negate an entire expression. This can be read as "if not."

Java program that uses not equals public class Program { public static void main(String[] args) { int value = 5; // This expression... if (value != 6) { System.out.println("Not 6!"); } // Is the same as this one. if (!(value == 6)) { System.out.println("Not 6!"); } } } Output Not 6! Not 6!

And, or. Often we chain expressions within an if-statement. We can use binary (two-part) operators for this. With && both expressions must evaluate to true.

And: With ||, only one expression must be true. After the first true evaluation, nothing further is tested.

Java program that uses and, or public class Program { public static void main(String[] args) { int width = 10; int height = 5; // Both expressions must evaluate to true. if (width == 10 && height == 5) { System.out.println("width 10 and height 5"); } // Only one expression must be true. if (width == 100 || height > 0) { System.out.println("width 100 or height greater than 0"); } } } Output width 10 and height 5 width 100 or height greater than 0

Boolean, store expressions. If-statements can become complex. Often an expression is repeated. We can store the result of an expression in a boolean, and then just test that.Boolean

Note: Imagine the "fits" boolean was tested many times. It would make the program simpler.

Note 2: If a time-consuming method call is part of an if, storing its result in a bool can help reduce calls (and improve performance).

Java program that uses boolean public class Program { public static void main(String[] args) { int width = 10; int height = 15; int weight = 200; // Use a boolean to store computed result. boolean fits = width <= 10 && height <= 20; // We can use the boolean, not a complex expression. if (fits && weight <= 150) { System.out.println("It fits"); } else { System.out.println("Does not fit"); } } } Output Does not fit

True and false. A boolean variable can equal true or false. We can test expressions and variables with the true or false keywords.

Exclamation: This means "not." So using !occupied will evaluate to true only if "occupied" is set to false.

Java program that uses true, false public class Program { public static void main(String[] args) { boolean vacant = true; boolean occupied = false; // Test boolean variables. if (vacant && !occupied) { System.out.println(true); } } } Output true

Method call. Often we call methods in if-statements. In this example we test for an odd number. We can read the if-statement as: "if is odd number."

Java program that uses method in if-statement public class Program { static boolean isOdd(int value) { // See if number is not evenly divisible by 2. return (value % 2) != 0; } public static void main(String[] args) { int number = 5; // Call method in if-statement. if (isOdd(number)) { System.out.println(true); } } } Output true

Reorder if-statements. Ifs are sequentially evaluated. The first expression is tested first. We can exploit this to optimize if-statement execution.

Here: We test a String against the literals "tea" and "java." The String equals "java" so evaluation continues until "java" is tested.

Version 1: This is the unoptimized version. It tests "tea" before "java" so two comparisons always occur.

Version 2: This version tests the most common condition first. So it incurs half as many total checks.

Tip: Check most common conditions first. We use heuristics, or frequency analysis, to optimize if-statements.

Java program that times if-statement reordering public class Program { public static void main(String[] args) { String value = "java"; int count = 0; long t1 = System.currentTimeMillis(); // Version 1: if-statements ordered most common last. for (int i = 0; i < 100000000; i++) { if (value == "tea") { count++; } else if (value == "java") { count += 2; } if (count == 0) { System.out.println(false); } } long t2 = System.currentTimeMillis(); // Version 2: if-statements ordered most common first. for (int i = 0; i < 100000000; i++) { if (value == "java") { count += 2; } else if (value == "tea") { count++; } if (count == 0) { System.out.println(false); } } long t3 = System.currentTimeMillis(); // ... Benchmark times. System.out.println(t2 - t1); System.out.println(t3 - t2); } } Output 35 ms: unoptimized order 32 ms: optimized order

Type mismatch. An expression in an if-statement must be evaluated to a boolean. In this program, we try to test an int, but this does not evaluate to true or false. It causes an error.

Java program that causes type mismatch public class Program { public static void main(String[] args) { int value = 1; // This does not compile: we must have a boolean expression. if (value) { System.out.println(1); } } } Output Exception in thread "main" java.lang.Error: Unresolved compilation problem: Type mismatch: cannot convert from int to boolean...

Switch. Another option is the switch-statement. With switch, we cannot use complex expressions, only constants. This is a slightly different concept.Switch

Ternary. In this kind of expression, we can assign a variable (or return a value) based on a condition. A ternary requires the question mark and ":" operators.Ternary Operator

A summary. The if-statement, and its friends else-if and else, are important constructs. In Java, they are part of nearly every program. We use them for simple and complex selections.

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Java if, else if, else Statements

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