Expressions and Statements

Overview

An expression has a value.
A statement describes a behavior. (It does not have a value.)
This lab lists a number of sample expressions and statements, and describes them in English. Try typing them in to the DrJava interactions pane and come up with your own experiments.

Why is it important to know about expressions and statements?
Because it helps us to:

Understand how the Java compiler "thinks" (understand Java's syntax rules).
Understand Java error messages so we can better debug programs.
Talk about the behavior of programs (e.g. when we're collaborating or asking for help).

Understand DrJava's behavior. If DrJava is given an expression, it prints the expression's value. For example:

> int x;
> x = 5   // this is an expression. 5 is assigned to x.
5         // when DrJava is given an expression, it prints the expression's value
> x = 6;  // this is a statement
>         // DrJava assigned 6 to x but didn't print 6 because it was given a 
          // statement (that was not a print statement)

Examples of Statements

For now, think of a statement as ending with a semicolon. Later we'll see that a statement can end with a block of code surrounded by curly braces.

Statements	Notes
Variable Declaraion Statements: int x; double d; char c; boolean result;	The behavior is that memory is allocated and a variable with a certain type is associated with the memory.
Assignment Statements: x = 500; d = 20.49 + 30.5 * 24; c = 'X'; result = false;	The behavior is that the value on the right is assigned to the variable on the left. NOTE: Assume we are using variables previously defined as above.
Combination Declaration/Assignment Statements: int x = 500; double d = 20.49 + 30.5 * 24; char c = 'X'; boolean result = false;	Combining declaration and initialization in one statementis preferred over having two separate statements. The behavior is the behavior of a declaration statement and an assignment statement.
Other Statements	We'll soon learn about other statements such as: if statements, if-else statements Loops: while loops, for loops, do-while loops etc.

Examples of Expressions

Expression	Value	Comments
Examples of literals:
5	5	The value of a literal is itself.
300.5	300.5
true	true
'b'	'b'
Expressions with arithmetic operators:
5 + 2	7
5 + 2 * 3	11	Why isn't it 21? Because, as in algebra, multiplication and division have higher precedence than addition and multiplication. You can use parentheses to group operands as you like. For example, the value of the expression (5 + 2) * 3 is 21. Recommended: use parentheses for clarity. See the Operator Precedence Table on the Resources page.
5 - 2 - 3	0	Why isn't it 6? Because, as in algebra, the arithmetic operators are left associative. You can use parentheses if you wan them to be grouped differently. For example, the value of the expression 5 - (2 - 3) is 6. See the Operator Precedence Table on the Resources page.
x = 500	500	The value of the expression is the value assigned.

Default Values

Instance variables that aren't assigned a value when they are declared are given a default value according to their type.