Slide #1.

Introducing Methods A method is a collection of statements that are grouped together to perform an operation. Define a method modifier method header return value type Invoke a method method name formal parameters public static int max(int num1, int num2) { int z = max(x, y); int result; method body if (num1 > num2) result = num1; else result = num2; } parameter list actual parameters (arguments) return value return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 1
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Slide #2.

Introducing Methods, cont. • Method signature is the combination of the method name and the parameter list. • The variables defined in the method header are known as formal parameters. • When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 2
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Slide #3.

Introducing Methods, cont. • A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void. For example, the returnValueType in the main method is void. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 3
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Slide #4.

Calling Methods pass the value of i public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } pass the value of j public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 4
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Slide #5.

animation Trace Method Invocation i is now 5 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 5
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Slide #6.

animation Trace Method Invocation j is now 2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 6
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Slide #7.

animation Trace Method Invocation invoke max(i, j) public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 7
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Slide #8.

animation Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 8
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Slide #9.

animation Trace Method Invocation declare variable result public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 9
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Slide #10.

animation Trace Method Invocation (num1 > num2) is true since num1 is 5 and num2 is 2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 10
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Slide #11.

animation Trace Method Invocation result is now 5 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 11
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Slide #12.

animation Trace Method Invocation return result, which is 5 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 12
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Slide #13.

animation Trace Method Invocation return max(i, j) and assign the return value to k public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 13
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Slide #14.

animation Trace Method Invocation Execute the print statement public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; System.out.println( "The maximum between " + i + " and " + j + " is " + k); } return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 14
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Slide #15.

CAUTION A return statement is required for a nonvoid method. The following method is logically correct, but it has a compilation error, because the Java compiler thinks it possible that this method does not return any value. public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; } To fix this problem, delete if (n<0) in the code. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 15
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Slide #16.

Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max). Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 16
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Slide #17.

animation Trace Call Stack i is declared and initialized public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } i: 5 The main method is invoked. return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 17
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Slide #18.

animation Trace Call Stack j is declared and initialized public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } j: 2 i: 5 The main method is invoked. return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 18
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Slide #19.

animation Trace Call Stack Declare k public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } Space required for the main method k: j: 2 i: 5 The main method is invoked. return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 19
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Slide #20.

animation Trace Call Stack Invoke max(i, j) public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } Space required for the main method k: j: 2 i: 5 The main method is invoked. return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 20
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Slide #21.

animation Trace Call Stack pass the values of i and j to num1 and num2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } return result; num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 21
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Slide #22.

animation Trace Call Stack pass the values of i and j to num1 and num2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } return result; result: num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 22
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Slide #23.

animation Trace Call Stack (num1 > num2) is true public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } return result; result: num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 23
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Slide #24.

animation Trace Call Stack Assign num1 to result public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } return result; Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 24
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Slide #25.

animation Trace Call Stack Return result and assign it to k public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } return result; Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k:5 j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 25
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Slide #26.

animation Trace Call Stack Execute print statement public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); } System.out.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; } Space required for the main method k:5 j: 2 i: 5 The main method is invoked. return result; Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 26
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Slide #27.

Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); } Suppose you invoke the method using nPrintln(“Welcome to Java”, 5); What is the output? Suppose you invoke the method using nPrintln(“Computer Science”, 15); What is the output? Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 27
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Slide #28.

Pass by Value The values of num1 and num2 are passed to n1 and n2. Executing swap does not affect num1 and num2. Space required for the swap method temp: n2: 2 n1: 1 Space required for the main method num2: 2 num1: 1 The main method is invoked Space required for the main method num2: 2 num1: 1 The swap method is invoked Space required for the main method num2: 2 num1: 1 The swap method is finished Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 Stack is empty The main method is finished 28
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Slide #29.

Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 29
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Slide #30.

Ambiguous Invocation public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 30
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Slide #31.

Scope of Local Variables A local variable: a variable defined inside a method. Scope: the part of the program where the variable can be referenced. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 31
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Slide #32.

Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different nonnesting blocks in a method, but you cannot declare a local variable twice in nested blocks. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 32
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Slide #33.

Scope of Local Variables, cont. A variable declared in the initial action part of a for loop header has its scope in the entire loop. But a variable declared inside a for loop body has its scope limited in the loop body from its declaration and to the end of the block that contains the variable. The scope of i The scope of j public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 33
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Slide #34.

Scope of Local Variables, cont. It is fine to declare i in two non-nesting blocks public static void method1() { int x = 1; int y = 1; It is wrong to declare i in two nesting blocks public static void method2() { int i = 1; int sum = 0; for (int i = 1; i < 10; i++) { x += i; } } for (int i = 1; i < 10; i++) { y += i; } for (int i = 1; i < 10; i++) { sum += i; } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 34
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Slide #35.

Scope of Local Variables, cont. // Fine with no errors public static void correctMethod() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 35
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Slide #36.

Scope of Local Variables, cont. // With no errors public static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rig hts reserved. 0-13-222158-6 36
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