Arrays
Why arrays are special
- There are three issues that distinguish arrays from other types of containers: efficiency, type, and the ability to hold primitives.
- The cost of this speed is that the size of an array object is fixed and cannot be changed for the lifetime of that array.
- You should generally prefer an ArrayList to an array.
- You’ll get a RuntimeException if you exceed the bounds, indicating a programmer error.
- Arrays are superior to pre-generic containers because you create an array to hold a specific type.
Arrays are first-class objects
- The array identifier is actually a reference to a true object that’s created on the heap. This is the object that holds the references to the other objects.
- length member that tells you how many elements can be stored in that array object.
Returning an array
- Returning an array is just like returning any other object—it’s a reference.
- The garbage collector takes care of cleaning up the array when you’re done with it, and the array will persist for as long as you need it.
Multidimensional arrays
- Each vector in the arrays that make up the matrix can be of any length.
- The Arrays.deepToString( ) method works with both primitive arrays and object arrays.
Arrays and generics
- You cannot instantiate arrays of parameterized types.
- Erasure removes the parameter type information, and arrays must know the exact type that they hold, in order to enforce type safety.
- You can parameterize the type of the array itself.
- The compiler won’t let you instantiate an array of a generic type. However, it will let you create a reference to such an array.
- Although you cannot create an actual array object that holds generics, you can create an array of the non-generified type and cast it.
- A generic container will virtually always be a better choice than an array of generics.
Creating test data
Arrays.fill()
- Since you can only call Arrays.fill( ) with a single data value, the results are not especially useful.
Data Generators
- If a tool uses a Generator, you can produce any kind of data via your choice of Generator.
Array Utilities
- There are six basic methods in Arrays: equals(), fill(), binarySearch(), sort(), toString(), hashCode().
Copying an array
- The Java standard library provides a static method, System.arraycopy( ), which can copy arrays.
- If you copy arrays of objects, then only the references get copied—there’s no duplication of the objects themselves.
Comparing arrays
- To be equal, the arrays must have the same number of elements, and each element must be equivalent to each corresponding element in the other array, using the equals( ) for each element.
Array element comparisons
- You hand a Strategy object to the code that’s always the same, which uses the Strategy to fulfill its algorithm.
- sort( ) casts its argument to Comparable.
Sorting an array
- The sorting algorithm that’s used in the Java standard library is designed to be optimal for the particular type you’re sorting—a Quicksort for primitives, and a stable merge sort for objects.
Searching a sorted array
- If you try to use binarySearchC ) on an unsorted array the results will be unpredictable.
- Otherwise, it produces a negative value representing the place that the element should be inserted if you are maintaining the sorted array by hand.
- If an array contains duplicate elements, there is no guarantee which of those duplicates will be found.