[Chapter 4] 4.6 Additive Operators
Author: Mark Grand
Date: Thu Jul 31 13:10:16 1997

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| Chapter 4 Expressions |

## 4.6 Additive Operators

The additive operators in Java are binary operators that are used for addition and string concatenation (`+`) and subtraction (`-`). The additive operators appear in additive expressions.

The additive operators are equal in precedence and are evaluated from left to right.

References Multiplicative Operators; Order of Operations

### Arithmetic Addition Operator +

The binary arithmetic addition operator `+` produces a pure value that is the sum of its operands. The arithmetic `+` operator may appear in an additive expression. The arithmetic addition operator never throws an exception.

Here is an example that uses the arithmetic addition operator:

    int addThree (int x, int y, int z) {
        return x + y + z;
    }

If the type of either operand of `+` is a reference to a `String` object, the operator is the string concatenation operator, not the arithmetic addition operator. The string concatenation operator is described in String Concatenation Operator +.

Otherwise, the types of both operands of the arithmetic addition operator must be arithmetic types, or a compile-time error occurs. The `+` operator may perform type conversions on its operands:

- If either operand is of type `double`, the other operand is converted to `double` and the operation produces a `double` value.

- Otherwise, if either operand is of type `float`, the other operand is converted to `float` and the operation produces a `float` value.

- Otherwise, if either operand is of type `long`, the other operand is converted to `long` and the operation produces a `long` value.

- Otherwise, both operands are converted to `int` and the operation produces an `int` value.

If the addition of integer data overflows, the value produced by `+` contains the low order bits of the sum and the sign of the value is the opposite of the mathematically correct sign, due to the limitations of the two's complement representation used for integer data.

The addition of floating-point data is governed by the following rules:

- If either operand is not-a-number (NaN), the sum is NaN.

- If one operand is positive infinity and the other is negative infinity, the sum is NaN.

- If both of the operands are positive infinity, the sum is positive infinity.

- If both of the operands are negative infinity, the sum is negative infinity.

- If one of the operands is an infinity value and the other operand is a finite value, the sum is the same infinity value as the operand.

- If one operand is positive zero and the other is negative zero; the sum is positive zero.

- If both operands are positive zero, the sum is positive zero.

- If both operands are negative zero, the sum is negative zero.

- If neither operand is NaN nor an infinity value, the sum is rounded to the nearest representable value. If the magnitude of the sum is too large to be represented, the operation overflows and an infinity value of the appropriate sign is produced. If the magnitude of the sum is too small to be represented, the operation underflows and a zero value of the appropriate sign is produced.

References Arithmetic Types; String Concatenation Operator +

### Arithmetic Subtraction Operator -

The binary subtraction operator `-` produces a pure value that is the difference between its operands; it subtracts its right operand from its left operand. The arithmetic `-` operator may appear in an additive expression. The arithmetic subtraction operator never throws an exception.

Here is an example that uses the arithmetic subtraction operator:

    int subThree (int x, int y, int z) {
        return x - y - z;
    }

The types of both operands of the arithmetic subtraction operator must be arithmetic types, or a compile-time error occurs. The `-` operator may perform type conversions on its operands:

- If either operand is of type `double`, the other operand is converted to `double` and the operation produces a `double` value.

- Otherwise, if either operand is of type `float`, the other operand is converted to `float` and the operation produces a `float` value.

- Otherwise, if either operand is of type `long`, the other operand is converted to `long` and the operation produces a `long` value.

- Otherwise, both operands are converted to `int` and the operation produces an `int` value.

For both integer and floating-point data, `a-b` always produces the same result as `a-(+b)`.

If the subtraction of integer data overflows, the value produced by `-` contains the low order bits of the difference and the sign of the value is the opposite of the mathematically correct sign, due to the limitations of the two's complement representation used for integer data.

The subtraction of floating-point data is governed by the following rules:

- If either operand is not-a-number (NaN), the difference is NaN.

- If the left operand is positive infinity and the right operand is negative infinity, the difference is positive infinity.

- If the left operand is negative infinity and the right operand is positive infinity, the difference is negative infinity.

- If both operands are positive infinity, the difference is NaN.

- If both operands are negative infinity, the difference is NaN.

- If the left operand is an infinity value and the right operand is a finite value, the difference is the same infinity value as the left operand.

- If the left operand is a finite value and the right argument is an infinity value, the difference is the opposite infinity value of the right operand.

- If both operands are either positive zero or negative zero, the difference is positive zero.

- If the left operand is positive zero and the right operand is negative zero, the difference is positive zero.

- If the left operand is negative zero and the right operand is positive zero, the difference is negative zero.

- If neither operand is NaN nor an infinity value, the difference is rounded to the nearest representable value. If the magnitude of the difference is too large to be represented, the operation overflows and an infinity value of the appropriate sign is produced. If the magnitude of the difference is too small to be represented, the operation underflows and a zero value of the appropriate sign is produced.

References Arithmetic Types

### String Concatenation Operator +

The string concatenation operator `+` produces a pure value that is a reference to a `String` object that it creates. The `String` object contains the concatenation of the operands; the characters of the left operand precede the characters of the right operand in the newly created string. The string concatenation `+` operator may appear in an additive expression.

Here is an example of some code that uses the string concatenation operator:

    // format seconds into hours, minutes, and seconds
    String formatTime(int t) {
        int minutes, seconds;
        seconds = t%60;
        t /= 60;
        minutes = t%60;
        return t/60 + ":" + minutes + ":" + seconds;
    }

If neither operand of `+` is a reference to a `String` object, the operator is the arithmetic addition operator, not the string concatenation operator. Note that Java does not allow a program to define overloaded operators. However, the language defines the `+` operator to have a meaning that is fundamentally different from arithmetic addition if at least one of its operands is a `String` object.

The way in which Java decides if `+` means arithmetic addition or string concatenation means that the use of parentheses can alter the meaning of the `+` operator. Consider the following code:

    int x = 3, y = 4;
    System.out.println("x = " + x + ", y = " + y);
    System.out.println("\"??\" + x + y ==> " + "??" + x + y);
    System.out.println("\"??\" + (x + y) ==> " + "??"+ (x + y));

In the output from this code, you can see that the addition of parentheses changes the meaning of the last `+` from string concatenation to arithmetic addition:

    x = 3, y = 4
    "??" + x + y ==> ??34
    "??" + (x + y) ==> ??7

If one of the operands of `+` is a `String` object and the other is not, the operand that is not a `String` object is converted to one using the following rules:

- If the operand is an object reference that is `null`, it is converted to the string literal `"null"`.

- If the operand is a non-`null` reference to an object that is not a string, the object's `toString()` method is called. The result of the conversion is the string value returned by the object's `toString()` method, unless the return value is `null`, in which case the result of the conversion is the string literal `"null"`. Since the `Object` class defines a `toString()` method, every class in Java has such a method.

- If the type of the operand is `char`, the operand is converted to a reference to a `String` object that has a length of one and contains that character.

- If the type of the operand is an integer type other than `char`, the operand is converted to a base 10 string representation of its value. If the value is negative, the string value starts with a minus sign; if it is positive there is no sign character. If the value is zero, the result of the conversion is `"0"`. Otherwise, the string representation of the integer does not have any leading zeros.

- If the type of the operand is a floating-point type, the exact string representation depends on the value being converted. If the absolute value of `d` is greater than or equal to 10^-3 or less than or equal to 10^7, it is converted to a string with an optional minus sign (if the value is negative) followed by up to eight digits before the decimal point, a decimal point, and the necessary number of digits after the decimal point (but no trailing zero if there is more than one significant digit). There is always a minimum of one digit after the decimal point.

  Otherwise, the value is converted to a string with an optional minus sign (if the value is negative), followed by a single digit, a decimal point, the necessary number of digits after the decimal point (but no trailing zero if there is more than one significant digit), and the letter `E` followed by a plus or a minus sign and a base 10 exponent of at least one digit. Again, there is always a minimum of one digit after the decimal point.

  In addition, the values `NaN`,`NEGATIVE_INFINITY`, `POSITIVE_INFINITY`, `-0.0`, and `+0.0` are represented by the strings `"NaN"`, `"--Infinity"`, `"Infinity"`, `"--0.0"`, and `"0.0"`, respectively.

  Note that the specification for this conversion has changed as of Java 1.1. Prior to that release, the conversion provided a string representation that was equivalent to the `%g` format of the `printf` function in C. In addition, the string representations of the infinity values, the zero values, and NaN are not specified prior to Java 1.1.

- If the type of the operand is `boolean`, the value is converted to a reference to either the string literal `"true"` or the string literal `"false"`.

Java uses the `StringBuffer` object to implement string concatenation. Consider the following code:

    String s, s1,s2;
    s = s1 + s2;

To compute the string concatenation, Java compiler generates code equivalent to:

    s = new StringBuffer().append(s1).append(s2).toString();

Consider another expression:

    s = 1 + s1 + 2

In this case, the Java compiler generates code equivalent to:

    s = new StringBuffer().append(1).append(s1).append(2).toString()

No matter how many strings are being concatenated in an expression, the expression always produces exactly one `StringBuffer` object and one `String` object.[3] From an efficiency standpoint, if you concatenate more than two strings, it may be more efficient to do so in a single expression, rather than in multiple expressions.

> [3] Although an optimizing compiler should be smart enough to combine multiple concatenation expressions when it is advantageous, the compiler provided with Sun's reference implementation of Java does not do this.

References Arithmetic Addition Operator +; Object; String; StringBuffer

Multiplicative Operators |  | Shift Operators