Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)

# Comparable

```
interface Comparable<T : Any!>
```

|---|
| [java.lang.Comparable](https://developer.android.com/reference/kotlin/java/lang/Comparable#) |

This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's *natural ordering* , and the class's method is referred to as its *natural comparison method*.

Lists (and arrays) of objects that implement this interface can be sorted automatically by `https://developer.android.com/reference/kotlin/java/util/Collections.html#sort(kotlin.collections.MutableList)` (and `https://developer.android.com/reference/kotlin/java/util/Arrays.html#sort(kotlin.Array)`). Objects that implement this interface can be used as keys in a [sorted map](https://developer.android.com/reference/kotlin/java/util/SortedMap.html#) or as elements in a [sorted set](https://developer.android.com/reference/kotlin/java/util/SortedSet.html#), without the need to specify a [comparator](https://developer.android.com/reference/kotlin/java/util/Comparator.html#).

The natural ordering for a class `C` is said to be *consistent with equals* if and only if `e1.compareTo(e2) == 0` has the same boolean value as `e1.equals(e2)` for every `e1` and `e2` of class `C`. Note that `null` is not an instance of any class, and `e.compareTo(null)` should throw a `NullPointerException` even though `e.equals(null)` returns `false`.

It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave "strangely" when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the `equals` method.

For example, if one adds two keys `a` and `b` such that `(!a.equals(b) && a.compareTo(b) == 0)` to a sorted set that does not use an explicit comparator, the second `add` operation returns false (and the size of the sorted set does not increase) because `a` and `b` are equivalent from the sorted set's perspective.

Virtually all Java core classes that implement `Comparable` have natural orderings that are consistent with equals. One exception is `https://developer.android.com/reference/kotlin/java/math/BigDecimal.html#`, whose [natural ordering](https://developer.android.com/reference/kotlin/java/math/BigDecimal.html#compareTo(java.math.BigDecimal)) equates `BigDecimal` objects with equal numerical values and different representations (such as 4.0 and 4.00). For `https://developer.android.com/reference/kotlin/java/math/BigDecimal.html#equals(kotlin.Any)` to return true, the representation and numerical value of the two `BigDecimal` objects must be the same.

For the mathematically inclined, the *relation* that defines the natural ordering on a given class C is:

```kotlin
<code>{(x, y) such that x.compareTo(y) &lt;= 0}.
  </code>
```
The *quotient* for this total order is:

```kotlin
<code>{(x, y) such that x.compareTo(y) == 0}.
  </code>
```
It follows immediately from the contract for `compareTo` that the quotient is an *equivalence relation* on `C`, and that the natural ordering is a *total order* on `C`. When we say that a class's natural ordering is *consistent with equals* , we mean that the quotient for the natural ordering is the equivalence relation defined by the class's `https://developer.android.com/reference/kotlin/java/lang/Object#equals(kotlin.Any)` method:

```kotlin
{(x, y) such that x.equals(y)}. 
```

In other words, when a class's natural ordering is consistent with equals, the equivalence classes defined by the equivalence relation of the `equals` method and the equivalence classes defined by the quotient of the `compareTo` method are the same.

This interface is a member of the [Java Collections Framework](https://developer.android.com//reference/java/util/package-summary.html#CollectionsFramework).

## Summary

| Public methods ||
|---|---|
| abstract [Int](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/index.html) | `https://developer.android.com/reference/kotlin/java/lang/Comparable#compareTo(java.lang.Comparable.T)(o: T)` Compares this object with the specified object for order. |

## Public methods

### compareTo

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)

```
abstract fun compareTo(o: T): Int
```

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

The implementor must ensure `https://developer.android.com/reference/kotlin/java/lang/Integer#signum(kotlin.Int)``(x.compareTo(y)) == -signum(y.compareTo(x))` for all `x` and `y`. (This implies that `x.compareTo(y)` must throw an exception if and only if `y.compareTo(x)` throws an exception.)

The implementor must also ensure that the relation is transitive: `(x.compareTo(y) > 0 && y.compareTo(z) > 0)` implies `x.compareTo(z) > 0`.

Finally, the implementor must ensure that `x.compareTo(y)==0` implies that `signum(x.compareTo(z)) == signum(y.compareTo(z))`, for all `z`.

| Parameters ||
|---|---|
| `o` | T: the object to be compared. |

| Return ||
|---|---|
| `https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/index.html` | a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object. |

| Exceptions ||
|---|---|
| `java.lang.ClassCastException` | if the specified object's type prevents it from being compared to this object. |
| `java.lang.NullPointerException` | if the specified object is null |