Array Initialization
int [] a1; int a2[]; |
declarations |
int [] a = new int[3]; |
a={0,0,0} |
int [] a = {1,2,3}; |
initializer list |
int [] a = new int[] {1,2,3}; |
initializer list |
int a[] = new int[]; |
compile error |
int [] a = new int[3] {1,2,3}; |
compile error |
Array length
System.out.print(a.length); //3
|
Compare:length for Array, length() for String and size() for ArrayList
Loop through Array
for (int i=0; i<arr.length; i++) {} //modify
for ( int a : arr) {...} //access only
|
Array as method parameter
passing an array is passing the memory address, the change on array content will be persistent through caller and callee. |
Array contents
int [] intArr={1,2,3}
int [] doubleArr=new double[3]
String [] objArr = new String[3]; //object type
|
Array Modification
Array is fixed in length once initialized, remove an element will resize the array and move all the element on its right. This could be computationally expensive. |
Arrays utilities
import java.util.Arrays;
Arrays.sort(arr);
Arrays.binarySearch(arr,item);
//if found, return index
//if not return -(potential index -1)
//if array not sorted,result is undefined
List list=Arrays.asList(arr);
Arrays.copyOfRange(Object[] src,from,to)
|
toString() method
not overloaded, inherited from Object class directly |
Variable arguments varargs
public static void main (String [] args){}
public static void main (String... args){}
//0 or more arguments
//similar to method overloading, or arrays
//args are treated as args[]
|
Multiple dimension array
//initialization, first index required
int [][] arr = new int[4][];
int [] arr[]= new int[5][];
int arr[][]= new int[6][]
int [] arr[][]= new int[5][][]; //3D
arr={{1,2,3},[4,5,6}};
//loop through
for (int i=0;i<arr.length;i++}{
for (int j=0;j<arr[i];j++) {... }
}
for (int innerArr:arr){
for (int num:innerArr){...}
}
|
|
|
ArrayList
import java.util.ArrayList; |
Size dynamically changed at runtime with autoshift |
last element indexed by size()-1 |
only contains reference type |
Array vs ArrayList
Array |
ArrayList |
length predefined |
size() variable |
manual shift |
Auto shift |
faster |
slower |
primitive+Ref type |
ref type only |
ArrayIndexOutOfBoundaryException |
IndexOutOfBoundaryException |
equals() not overriden |
overriden equals() |
toString() not overriden |
overriden toString() |
Arrays.sort() |
Collections.sort() |
Arrays.binarySearch() |
Collections.binarySearch() |
Create ArrayList
ArrayList aL1 = new ArrayList();
ArrayList aL2 = new ArrayList(5);
ArrayList aL3 = new ArrayList(aL1);
ArrayList<String> aL4,aL5;
aL4=new ArrayList<String>(); //Java5
aL5 = new ArrayList<>(); //Java 7
ArrayList aL6 = Arrays.asList(arr);
|
IllegArgumentException // when primary type value assigned
Using ArrayList
add |
Boolean add(E element) |
| |
void add (int index, E element) |
remove |
Boolean remove(Object obj) |
| |
E remove(int index) |
set |
E set(int index, E element) |
size |
int size() |
| |
boolean isEmpty() |
clear |
void clear() |
contains |
boolean contains(Object obj) |
equals |
boolean equals(Object obj) |
Wrraper class
To put primitive type to ArrayList, using wrapper class |
Boolean,Byte,Short,Integer,Long,Float,Double,Character |
Xxx.parseXxx(String) //convnert string to a primitive |
Xxx.valueOf(String) //convert string to a wrapper class(Xxx) |
NumberFormatException
|
|
Autoboxing
Auto primitive value > wrapper class obj |
null allowed boxing into list, print ok,but unboxing is not allowed --> NullPointexception |
Integer List is interest: int as index is precedent autoboxing. remove(1) will remove second element, not Integer(1) |
Convert Between Array and List
1. toArray()
listObj.toArray(); //convert to obj arr
listObj.toArray(new String[0]);
//index= 0 will return a new string array,
//index >0 will reuse if fit, else return new one
2. AsList()
//get a list with fixed size
list =Arrays.asList(arrayObj)
list.remove(1): //UnsupportedOperation
List<String> args=Arrays.asList("one","two");//varargs
// get a new arrayList obj,
ArrayList<String> aL=new ArrayList(Arrays.asList("one","two"));
|
toArray() is list object method, return new array object
asList() is Arrays class static method, return the memory location as a list (same structure shared by array and list, content can be changed by both array and list,size still fixed)
|
