[Chapter 4] 4.9 Equality Comparison Operators
Author: Mark Grand
Date: Thu Jul 31 13:10:29 1997

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| Chapter 4 Expressions |

## 4.9 Equality Comparison Operators

The equality comparison operators in Java are used for equal-to (`==`) and not-equal-to (`!=`) comparison operations. The equality comparison operators may appear in an equality expression:

The equality comparison operators are equal in precedence and are evaluated from left to right. The `==` and `!=` comparison operators can perform numerical comparisons, `boolean` comparisons, and reference type comparisons. Both of these operators produce `boolean` values.

References Relational Comparison Operators; Order of Operations

### Equal-To Operator ==

The equal-to operator `==` performs a comparison between its operands and returns a `boolean` value. It returns the pure value `true` if the operands are equal to each other; otherwise it returns the pure value `false`. The `==` operator may appear as part of an equality expression. The equal-to operator never throws an exception.

The operands of `==` may be of any type, but they must both be of the same kind of type or a compile-time error occurs. If one operand is of an arithmetic type, the other must also be of an arithmetic type. If one operand is of type `boolean`, the other must also be of type `boolean`. If one operand is a reference type, the other must also be a reference type. Note that neither operand can be an expression that invokes a `void` method.

If both operands are of arithmetic types, then the operator performs an arithmetic equality comparison. The operator may perform type conversions on the operands:

- If either operand is of type `double`, then the other operand is converted to `double`.

- Otherwise, if either operand is of type `float`, the other operand is converted to `float`.

- Otherwise, if either operand is of type `long`, the other operand is converted to `long`.

- Otherwise, both operands are converted to `int`.

The equality comparison of any two arithmetic values produces `true` if and only if both operands are the same value; otherwise the comparison produces `false`. The comparison of floating-point data is governed by the following additional rules:

- If either operand is not-a-number (NaN), the comparison produces `false`.

- Positive infinity is a distinct value that is equal to itself, and not equal to any other value.

- Negative infinity is a distinct value that is equal to itself, and not equal to any other value.

- Positive and negative zero are treated as equal, so `-0.0==0.0` produces `true`.

If both operands are `boolean` values, the operator performs a Boolean equality comparison. The comparison produces `true` if both operands are `true` or both operands are `false`. Otherwise, the comparison produces `false`.

If both operands are reference types, the operator performs an object equality comparison. In order to perform this type of comparison, it must be possible to cast the value of one of the operands to the type of the other operand, or a compile-time error occurs. The comparison produces `true` if both of its operands refer to the same object or if both of its operands are `null`; otherwise the comparison produces `false`.

Because the `==` operator determines if two objects are the same object, it is not appropriate for comparisons that need to determine if two objects have the same contents. For example, if you need to know whether two `String` objects contain the same sequences of characters, the `==` operator is inappropriate. You should use the `equals()` method instead:[4]

> [4] This is similar to the difference in C between writing `string1==string2` and `strcmp(string1, string2)==0`.

    string1.equals (string2)  // Compares contents of strings
    string1 == string2        // Compares actual string objects

References Arithmetic Types; Boolean Type; Reference Types

### Not-Equal-To-Operator !=

The not-equal-to operator `!=` performs a comparison between its operands and returns a `boolean` value. It returns the pure value `true` if the operands are not equal to each other; otherwise it returns the pure value `false`. The `!=` operator may appear as part of an equality expression. The not-equal-to operator never throws an exception.

The operands of `!=` may be of any type, but they must both be of the same kind of type or a compile-time error occurs. If one operand is of an arithmetic type, the other must also be of an arithmetic type. If one operand is of type `boolean`, the other must also be of type `boolean`. If one operand is a reference type, the other must also be a reference type. Note that neither operand can be an expression that invokes a `void` method.

If both operands are of arithmetic types, the operator performs an arithmetic inequality comparison. The operator may perform type conversions on the operands:

- If either operand is of type `double`, then the other operand is converted to `double`.

- Otherwise, if either operand is of type `float`, the other operand is converted to `float`.

- Otherwise, if either operand is of type `long`, the other operand is converted to `long`.

- Otherwise, both operands are converted to `int`.

The inequality comparison of any two arithmetic values produces `true` if and only if both operands are not the same value; otherwise the comparison produces `false`. The comparison of floating-point data is governed by the following additional rules:

- If either operand is not-a-number (NaN), the comparison produces `true`. NaN is the only value that compares as not equal to itself.

- Positive infinity is a distinct value that is equal to itself, and not equal to any other value.

- Negative infinity is a distinct value that is equal to itself, and not equal to any other value.

- Positive and negative zero are treated as equal, so `-0.0!=0.0` produces `false`.

If both operands are `boolean` values, the operator performs a Boolean inequality comparison. The comparison produces `false` if both operands are `true` or both operands are `false`. Otherwise, the comparison produces `true`.

If both operands are reference types, the operator performs an object equality comparison. In order to perform this type of comparison, it must be possible to cast the value of one of the operands to the type of the other operand, or a compile-time error occurs. The comparison produces `true` if both of its operands refer to different objects and if both of its operands are not `null`; otherwise the comparison produces `false`.

Because the `!=` operator determines if two objects are different objects, it is not appropriate for comparisons that need to determine if two objects have different contents. For example, if you need to know whether two `String` objects contain different sequences of characters, the `!=` operator is inappropriate. You should use the `equals()` method instead:[5]

> [5] This is similar to the difference in C between writing `string1!=string2` and `strcmp(string1, string2)!=0`.

    !string1.equals (string2) // Compares contents of strings
    string1 != string2        // Compares actual string objects

References Arithmetic Types; Boolean Type; Reference Types

Relational Comparison Operators |  | Bitwise/Logical Operators