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The assignment expressions

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• The assignment operator in an assignment statement
  • We have seen the assignment statement:

    var = expr;

    Effect:

    Stores the value of expression expr in the variable named var

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• The assignment expression
  • Consider the following arithmetic expression:

    int a = 3; int b = 4; Arithmetic expression: a + b Returns the result: 7

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  • Definition: assignment expression:

    var = expr

    Result returned by an assignment expression:

    Result of var = expr is equal to the value of expr

    Note:

    The assignment operator will (still) update the value stored inside the variable var In addition to updating the variable. the assignment operator also returns a value

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  • Example:

    public class AssignExpr01 { public static void main(String[] args) { int a; a = 0; System.out.println(a); System.out.println(a = 2); // Prints 2 System.out.println(a); // Prints 2 } }

    Explanation:

    System.out.println(a = 2) will print the value between its brackets The expression between the brackets is: a = 2 which is an assignment expression The assignment expression returns the value 2 Therefore, System.out.println will print the value 2

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  • Example Program: (See the result yourself)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac AssignExpr01.java To run:          java AssignExpr01

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• The result returned by the assignment operators
  • We have seen many different assignment operators:

    = +=          -= *= /= %=

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  • Result returned by the assignment operators:

    Operator	Example	Returned value
    =	a = 2	2
    +=	a += 2	a + 2
    -=	a -= 2	a - 2
    *=	a *= 2	a * 2
    /=	a /= 2	a / 2
    %=	a %= 2	a % 2

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  • Example:

    public class AssignExpr02 { public static void main(String[] args) { int a; a = 5; System.out.println(a = 2); a = 5; System.out.println(a += 2); a = 5; System.out.println(a -= 2); a = 5; System.out.println(a *= 2); a = 5; System.out.println(a /= 2); a = 5; System.out.println(a %= 2); } }

    Output:

    2 7 3 10 2 1

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  • Example Program: (Try it out for yourself...)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac AssignExpr02.java To run:          java AssignExpr02

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• Priority and associativity of the assignment operators
  • Priority of the assignment operators:

    All of the assignment operators have the lowest priority among all operators in Java

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  • Associativity of the assignment operators:

    All of the assignment operators are right associative Meaning: When there are multiple assignment operators in an expression, the expression is evaluate from right to left

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  • Example: cascaded assignment

    public class AssignExpr03 { public static void main(String[] args) { int a, b, c; c = b = a = 4 + 2; // Cascade assignment System.out.println(a); // Prints 6 System.out.println(b); // Prints 6 System.out.println(c); // Prints 6 } }

    Explanation:

    c = b = a = 4 + 2; is evaluated as follows: c = b = a = 4 + 2; higher priority ^^^^^ Reduces to: c = b = a = 6; from right to left ^^^^^ assigns 6 to variable a and returns 6 Reduces to: c = b = 6; from right to left ^^^^^ assigns 6 to variable b and returns 6 Reduces to: c = 6; assigns 6 to variable c (and returns 6) ^^^^^

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  • Example Program: (Try it out yourself)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac AssignExpr03.java To run:          java AssignExpr03

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• Exercise
  • What will the following Java program print:

    public class AssignExpr04 { public static void main(String[] args) { int a, b, c; a = 1; b = 1; c = 2; c *= b -= a += 1 + 2; System.out.println(a); System.out.println(b); System.out.println(c); } }

    Answer:

    4 -3 -6

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  • Example Program: (Verify for yourself...)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac AssignExpr04.java To run:          java AssignExpr04