Java Expressions
Expressions in Java are the primary construct for doing calculations.
Remarks[edit  edit source]
For a reference on the operators that can be used in expressions, see Operators.
Operator Precedence[edit  edit source]
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:
 Add
1
and2
and multiply the result by3
. This gives the answer9
. If we added parentheses, this would look like( 1 + 2 ) x 3
.  Add
1
to the result of multiplying2
and3
. This gives the answer7
. If we added parentheses, this would look like1 + ( 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 "selfevident" (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.
Description  Operators / constructs (primary)  Precedence  Associativity  

Qualifier
Parentheses Instance creation Field access Array access Method invocation Method reference 
name. name
primary primary primary primary 
15  Left to right  
Post increment  expr++ , expr

14    
Pre increment
Unary Cast^{1} 
++ expr,  expr,

13  
Right to left Right to left  
Multiplicative  * / %  12  Left to right  
Additive  +   11  Left to right  
Shift  << >> >>>  10  Left to right  
Relational  < > <= >= instanceof

9  Left to right  
Equality  == !=  8  Left to right  
Bitwise AND  &  7  Left to right  
Bitwise exclusive OR  ^  6  Left to right  
Bitwise inclusive OR  5  Left to right  
Logical AND  &&  4  Left to right  
Logical OR  3  Left to right  
Conditional^{1}  ? :  2  Right to left  
Assignment
Lambda^{1} 
=
> 
1  Right to left 
^{1} Lambda expression precedence is complex, as it can also occur after a cast, or as the third part of the conditional ternary operator.
Expression evaluation order[edit  edit source]
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[edit  edit source]
In the following example:
int i = method1() + method2();
the order of evaluation is:
 The left operand of
=
operator is evaluated to the address ofi
.  The left operand of the
+
operator (method1()
) is evaluated.  The right operand of the
+
operator (method2()
) is evaluated.  The
+
operation is evaluated.  The
=
operation is evaluated, assigning the result of the addition toi
.
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 sideeffect[edit  edit source]
In the following example:
int i = 1;
intArray[i] = ++i + 1;
the order of evaluation is:
 The left operand of
=
operator is evaluated. This gives the address ofintArray[1]
.  The preincrement is evaluated. This adds
1
toi
, and evaluates to2
.  The right hand operand of the
+
is evaluated.  The
+
operation is evaluated to:2 + 1
>3
.  The
=
operation is evaluated, assigning3
tointArray[1]
.
Note that since the lefthand operand of the =
is evaluated first, it is not influenced by the sideeffect of the ++i
subexpression.
Reference:
Expression Basics[edit  edit source]
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 typecast 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
 Simple identifiers; e.g.
 Primaries which consist of:
 Literals; e.g.
1
,1.0
,'X'
,"hello"
,false
andnull
 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
orthis.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
 Literals; e.g.
 Unary operator expressions; e.g.
!a
ori++
 Binary operator expressions; e.g.
a + b
orobj == 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[edit  edit source]
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 standalone 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[edit  edit source]
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[edit  edit source]
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 operationandbecomes assignments.
 Pre and post increment and decrement expressions.
 Method calls (
void
or nonvoid
).  Class instance creation expressions.
Constant Expressions[edit  edit source]
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 FPstrict semantics.
Uses for Constant Expressions[edit  edit source]
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.