Summary of common methods of ArrayList

aggregate

Concept ：

Collections are also called containers , be used for Storage 、 extract 、 Delete data .（ That is, increase , Delete , lookup ）.JDK Provided collection API All contained in java.util In bag .
It's like an array .

Branch of set ：

The relationship is as shown in the figure ：

ArrayList

Common methods ：

1.add()

effect ：
Used to direct to List Add elements to the collection container .
Is a generic ： Used to constrain the data types of elements in the collection container , If omitted , The default is Object type , because Object Is the parent of all classes .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		System.out.println(list);
	}
}

result ：

[Tom]

2.size()

effect ：
Used to get how many elements are in the collection .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		int length=list.size();
		System.out.println(length);
	}
}

result ：

1

3.get();

effect ：

Gets the specified index （ from 0 Start ） The element of location .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		System.out.println(list.get(2));
	}
}

result ：

array

4.add( __, " ");

effect ：

Add the element in the specified position , The element in its original position moves back .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.add(0, "Kate");
		System.out.println(list.get(2));
	}
}

result ：

lili

5.set(int i, Object element)

effect ：

Replace the element at the specified location .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		System.out.println(list.get(2));
	}
}

result ：

Lucy

6.clear()

effect ：

Empty List All elements in the collection .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		System.out.println(list.get(2));
		list.clear();
		System.out.println(list);
	}
}

result ：

Lucy
[]

7.isEmpty()

effect ：

Used to determine whether there are elements in the collection container , Returns a Boolean type .

If No, , be true.

Yes Then for flase.

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		System.out.println(list.get(2));
		boolean flag = list.isEmpty();
		System.out.println(flag);
		list.clear();
		System.out.println(list);
		boolean flag2 = list.isEmpty();
		System.out.println(flag2);
	}
}

result ：

Lucy
false
[]
true

8.contains(Object o)

effect ：

Used to judge whether the collection container contains all parameter elements .
Without this element, it returns flase.
With the return true.

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		boolean flag = list.contains("Jim");
		System.out.println(flag);
	}
}

result ：

false

reflection ： What is the judgment process ？
We can hold Ctrl Click to enter contains Inside , Get the internal code as ：

public boolean contains(Object o) {
    
        return indexOf(o) >= 0;
    }
public int indexOf(Object o) {
    
        if (o == null) {
    
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
    
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

o That is, the element you want to find jim. Because after entering o Defined as Object type , So at this time, he is the object of transformation . Because the element searched is not empty , So to enter else Branch . Get into if When the sentence is ,o.equals(elementData[i]) This code looks like the calling parent Object The sentence in , It's actually called String In type . This forms a polymorphism .
Polymorphism is to make it find all types , Because not all of your collections are of a specific type .

9.remove()

effect ：

Delete specified index （ from 0 Start ） The element of location , And return the element to , And the following elements move forward .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		String str = list.remove(0);
		System.out.println(list);
	}
}

result ：

[lili, Lucy]

10.remove(Object o)

effect ：

Delete List In the collection xx Elements , return boolean type , And move the following elements forward .

notes ：
Attention comparison 9 and 10 The difference between .
example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		String str = list.remove(0);
		System.out.println(list);
		list.remove("lili");
		System.out.println(list.get(0));
	}
}

result ：

[lili, Lucy]
Lucy

11.iterator()

effect ：

take List Elements in the collection go to iterator variable

principle ：

Judge the present “ The pointer ” Are there any other elements below , If there is an element under the pointer , Then move the pointer and get the element at the corresponding position .

example ：

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		String str = list.remove(0);
		System.out.println(list);
		list.remove("lili");
		System.out.println(list.get(0));
		Iterator<String> iterator = list.iterator();
		while(iterator.hasNext()) {
    
			System.out.println(iterator.next());
		}	
	}
}

result ：

[lili, Lucy]
Lucy
Lucy

Traversal of the set

Three methods ：

The first one is ： Conventional methods

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		for(int i=0;i<list.size();i++) {
    
			System.out.println(list.get(i));
		}
	}
}

The second kind ：for-each

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		for(String name:list) {
    
			System.out.println(name);
		}
	}
}

The third kind of ： utilize iterator Method

package list;
import java.util.ArrayList;
import java.util.Iterator;
public class Test {
    
	public static void main(String [] args) {
    	
		ArrayList <String>list =new ArrayList<String>(); 
		list.add("Tom");
		list.add("lili");
		list.add("array");
		list.set(2, "Lucy");
		Iterator<String> iterator = list.iterator();
		while(iterator.hasNext()) {
    
			System.out.println(iterator.next());
		}
	}
}