Expressions

Introduction#

Expressions in Java are the primary construct for doing calculations.

Remarks#

For a reference on the operators that can be used in expressions, see Operators.

Operator Precedence

When an expression contains multiple operators, it can potentially be read in different ways. For example, the mathematical expression 1 + 2 x 3 could be read in two ways:

1. Add 1 and 2 and multiply the result by 3. This gives the answer 9. If we added parentheses, this would look like ( 1 + 2 ) x 3.
2. Add 1 to the result of multiplying 2 and 3. This gives the answer 7. If we added parentheses, this would look like 1 + ( 2 x 3 ).

In mathematics, the convention is to read the expression the second way. The general rule is that multiplication and division are done before addition and subtraction. When more advanced mathematical notation is used, either the meaning is either “self-evident” (to a trained mathematician!), or parentheses are added to disambiguate. In either case, the effectiveness of the notation to convey meaning depends on the intelligence and shared knowledge of the mathematicians.

Java has the same clear rules on how to read an expression, based on the precedence of the operators that are used.

In general, each operator is ascribed a precedence value; see the table below.

For example:

  1 + 2 * 3

The precedence of + is lower than the precedence of *, so the result of the expression is 7, not 9.

¹ Lambda expression precedence is complex, as it can also occur after a cast, or as the third part of the conditional ternary operator.

Constant Expressions

A constant expression is an expression that yields a primitive type or a String, and whose value can be evaluated at compile time to a literal. The expression must evaluate without throwing an exception, and it must be composed of only the following:

• Primitive and String literals.

• Type casts to primitive types or String.

• The following unary operators: +, -, ~ and !.

• The following binary operators: *, /, %, +, -, <<, >>, >>>, <, <=, >, >=, ==, !=, &, ^, |, && and ||.

• The ternary conditional operator ? :.

• Parenthesized constant expressions.

• Simple names that refer to constant variables. (A constant variable is a variable declared as final where the initializer expression is itself a constant expression.)

• Qualified names of the form <TypeName> . <Identifier> that refer to constant variables.

Note that the above list excludes ++ and --, the assignment operators, class and instanceof, method calls and references to general variables or fields.

Constant expressions of type String result in an “interned” String, and floating point operations in constant expressions are evaluated with FP-strict semantics.

Uses for Constant Expressions

Constant expressions can be used (just about) anywhere that a normal expression can be used. However, they have a special significance in the following contexts.

Constant expressions are required for case labels in switch statements. For example:

switch (someValue) {
case 1 + 1:            // OK
case Math.min(2, 3):   // Error - not a constant expression
    doSomething();
}

When the expression on the right hand side of an assignment is a constant expression, then the assignment can perform a primitive narrowing conversion. This is allowed provided that the value of the constant expression is within the range of the type on the left hand side. (See JLS 5.1.3 and 5.2) For example:

byte b1 = 1 + 1;             // OK - primitive narrowing conversion.
byte b2 = 127 + 1;           // Error - out of range
byte b3 = b1 + 1;            // Error - not a constant expession
byte b4 = (byte) (b1 + 1);   // OK

When a constant expression is used as the condition in a do, while or for, then it affects the readability analysis. For example:

while (false) {
    doSomething();           // Error - statenent not reachable
}
boolean flag = false;
while (flag) {
    doSomething();           // OK
}

(Note that this does not apply if statements. The Java compiler allows the then or else block of an if statement to be unreachable. This is the Java analog of conditional compilation in C and C++.)

Finally, static final fields in an class or interface with constant expression initializers are initialized eagerly. Thus, it is guaranteed that these constants will be observed in the initialized state, even when there is a cycle in the class initialization dependency graph.

For more information, refer to JLS 15.28. Constant Expressions.

Expression evaluation order

Java expressions are evaluated following the following rules:

• Operands are evaluated from left to right.
• The operands of an operator are evaluated before the operator.
• Operators are evaluated according to operator precedence
• Argument lists are evaluated from left to right.

Simple Example

In the following example:

int i = method1() + method2();

the order of evaluation is:

1. The left operand of = operator is evaluated to the address of i.
2. The left operand of the + operator (method1()) is evaluated.
3. The right operand of the + operator (method2()) is evaluated.
4. The + operation is evaluated.
5. The = operation is evaluated, assigning the result of the addition to i.

Note that if the effects of the calls are observable, you will be able to observe that the call to method1 occurs before the call to method2.

Example with an operator which has a side-effect

In the following example:

int i = 1;
intArray[i] = ++i + 1;

the order of evaluation is:

1. The left operand of = operator is evaluated. This gives the address of intArray[1].
2. The pre-increment is evaluated. This adds 1 to i, and evaluates to 2.
3. The right hand operand of the + is evaluated.
4. The + operation is evaluated to: 2 + 1 -> 3.
5. The = operation is evaluated, assigning 3 to intArray[1].

Note that since the left-hand operand of the = is evaluated first, it is not influenced by the side-effect of the ++i subexpression.

Reference:

• JLS 15.7 - Evaluation Order

Expression Basics

Expressions in Java are the primary construct for doing calculations. Here are some examples:

1                 // A simple literal is an expression
1 + 2             // A simple expression that adds two numbers
(i + j) / k       // An expression with multiple operations
(flag) ? c : d    // An expression using the "conditional" operator
(String) s        // A type-cast is an expression
obj.test()        // A method call is an expression
new Object()      // Creation of an object is an expression
new int[]         // Creation of an object is an expression

In general, an expression consists of the following forms:

• Expression names which consist of:
  • Simple identifiers; e.g. someIdentifier
  • Qualified identifiers; e.g. MyClass.someField
• Primaries which consist of:
  • Literals; e.g. 1, 1.0, 'X', "hello", false and null
  • Class literal expressions; e.g. MyClass.class
  • this and <TypeName> . this
  • Parenthesized expressions; e.g. ( a + b )
  • Class instance creation expressions; e.g. new MyClass(1, 2, 3)
  • Array instance creation expressions; e.g. new int[3]
  • Field access expressions; e.g. obj.someField or this.someField
  • Array access expressions; e.g. vector[21]
  • Method invocations; e.g. obj.doIt(1, 2, 3)
  • Method references (Java 8 and later); e.g. MyClass::doIt
• Unary operator expressions; e.g. !a or i++
• Binary operator expressions; e.g. a + b or obj == null
• Ternary operator expressions; e.g. (obj == null) ? 1 : obj.getCount()
• Lambda expressions (Java 8 and later); e.g. obj -> obj.getCount()

The details of the different forms of expressions may be found in other Topics.

• The Operators topic covers unary, binary and ternary operator expressions.
• The Lambda expressions topic covers lambda expressions and method reference expressions.
• The Classes and Objects topic covers class instance creation expressions.
• The Arrays topic covers array access expressions and array instance creation expressions.
• The Literals topic covers the different kinds of literals expressions.

The Type of an Expression

In most cases, an expression has a static type that can be determined at compile time by examining and its subexpressions. These are referred to as stand-alone expressions.

However, (in Java 8 and later) the following kinds of expressions may be poly expressions:

• Parenthesized expressions
• Class instance creation expressions
• Method invocation expressions
• Method reference expressions
• Conditional expressions
• Lambda expressions

When an expression is a poly expression, its type may be influenced by the expression’s target type; i.e. what it is being used for.

The value of an Expression

The value of an expression is assignment compatible with its type. The exception to this is when heap pollution has occurred; e.g. because “unsafe conversion” warnings have been (inappropriately) suppressed or ignored.

Expression Statements

Unlike many other languages, Java does not generally allow expressions to be used as statements. For example:

public void compute(int i, int j) {
    i + j;   // ERROR
}

Since the result of evaluating an expression like cannot be use, and since it cannot affect the execution of the program in any other way, the Java designers took the position that such usage is either a mistake, or misguided.

However, this does not apply to all expressions. A subset of expressions are (in fact) legal as statements. The set comprises:

• Assignment expression, including operation-and-becomes assignments.
• Pre and post increment and decrement expressions.
• Method calls (void or non-void).
• Class instance creation expressions.