03-ArrayList

Properties

• all elements have to be reference type
  • String
  • class
  • Integer / Double / Boolean
• size is changeable
• 可以直接打印（array不行）

Declaration

example:

ArrayList<String> names = new ArrayList<String>();

Methods

This part is not an exhaustive list of all the methods but a note on important things you should focus on. To see methods that may appear on the test, refer to the Java Quick Reference

add()

valid index: [0, size]

get() and set()

valid index: [0, size - 1]

Comparison of String, Array, and Arraylist

Type	Get the length	Get an element
String	str.length()	substring(i, i+1)
Array	arr.length (this is an attribute, not a method, so no brackets after it)	arr[i]
Arraylist	name.size()	get(i)

Common Algorithms

max and min

• array

double largest = values[0];
for(int i = 1; i < values.length; i++){
	if(values[i] > largest){
	largest = values[i];
	}
}

• arraylist

double largest = values.get(0);
for(int i = 1; i < values.size(); i++){
	if(values.get(i) > largest){
	largest = values.get(i);
	}
}

deleting elements

Deleting elements during a traversal of an ArrayList requires using special techniques to avoid skipping elements. There are 3 ways:

1. if-else

• use while loop cuz the number of loops is not fixed
• if removed, index doesn’t ++ ⇒ check same index (previously at the next index)
• if not removed, move on (index ++)

//remove all strings of length < 4 from an array list
int i = 0;
while (i < words.size()){
	String word = words.get(i);
	if (word.length() < 4){
		words.remove(i);
	}
	else{
		i++;
	}
}

2. i--

• if removed, index —

//remove all strings of length < 4 from an array list
for(int i = 0;i<names.size();i++){
	String name = names.get(i);
	if(name.length()<4){
		names.remove(i);
		i--;
	}
}

3. traverse from end to start

//remove all strings of length < 4 from an array list
for(int i = names.size() - 1; i >= 0; i--){
	String name = names.get(i);
	if(name.length() < 4){
	names.remove(i);
	}
}

Compare