如何使用Java流获取字符串和存储在类对象的Arraylist中的ArrayList

Question

I have modified the code and trying to get an ArrayList and the String stored in an Arraylist of Objects on a specific condition(say 'str' string equal to 2). I'm not able to convert the Stream to ArrayList. Please help me understand what needs to be done to get the ArrayList from this stream.

I have a class 'SampleClass' like below:

import java.util.ArrayList;

public class SampleClass {
String str;
ArrayList<String> al;
String check;

public SampleClass(String str, ArrayList<String> al, String check) {
    super();
    this.str = str;
    this.al = al;
    this.check = check;
}
public String getStr() {
    return str;
}
public void setStr(String str) {
    this.str = str;
}
public ArrayList<String> getAl() {
    return al;
}
public void setAl(ArrayList<String> al) {
    this.al = al;
}
public String getCheck() {
    return check;
}
public void setCheck(String check) {
    this.check = check;
}

}

I have another class 'GetTheArrayListStoredInAnotherArrayList' like below where I'm trying to get the ArrayList stored inside the ArrayList of objects. Please correct me where I'm wrong.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.stream.Collectors;

public class GetTheArrayListStoredInAnotherArrayList{

public static void main(String[] args) {
    String test = "qw,rer,try,try,erh5,wertgw45t,45";
    ArrayList<String> al = new ArrayList<String>();
    al.addAll(new ArrayList<String>(Arrays.asList(test.split(","))));
    System.out.println(al);
    ArrayList<SampleClass> sca = new ArrayList<SampleClass>();
    SampleClass sc1 = new SampleClass("1", al,"ch1");
    SampleClass sc2 = new SampleClass("2", al,"cc2");
    SampleClass sc3 = new SampleClass("3", al,"fr3");
    SampleClass sc4 = new SampleClass("4", al,"fg4");
    sca.add(sc1);
    sca.add(sc2);
    sca.add(sc3);
    sca.add(sc4);
    ArrayList<String> als1 = null;
    ArrayList<String> als = sca.stream().filter( s -> s.getStr().equals("2")).flatMap(sc -> sc.getAl().stream()).collect(Collectors.toCollection(ArrayList::new));
    System.out.println(als);
    String ch = (String) sca.stream().filter(s -> s.getStr().equals("1")).map(ac -> ac.getCheck());
    System.out.println(ch);
}
}

I got the below error when I executed the code :

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
Cannot cast from Stream<String> to String

at GetTheArrayListStoredInAnotherArrayList.main(GetTheArrayListStoredInAnotherArrayList.java:24)

Answer 1

Not entirely sure what you are trying to do, but you need to change your code a bit:

         List<String> als = sca.stream()
             .filter(s -> s.getStr().equals("2"))
             .flatMap(sc -> sc.getAl().stream())
             .collect(Collectors.toList());

A few things :

flatMap must return a Stream (in your case you are returning a List )

Collectors.toList makes no guarantee of the List in returns, so the assignment is to a List , not an ArrayList .

EDIT

This:

  Stream<String> stream = sca.stream().filter(s -> s.getStr().equals("1"))
                                   .map(ac -> ac.getCheck());

Will produce a Stream<String> . You can't simply cast that to a String , you have to collect/reduce that to whatever you want. Like let's say a List :

  List<String> list = sca.stream()
        .filter(s -> s.getStr().equals("1"))
        .map(ac -> ac.getCheck())
        .collect(Collectors.toList());

Or a single String for example:

  String r = sca.stream()
        .filter(s -> s.getStr().equals("1"))
        .map(ac -> ac.getCheck())
        .collect(Collectors.joining(","));

This is actually basic stuff... you should really study some samples and the documentation.

Answer 2

Change

ArrayList<String> als = sca.stream().filter( s -> s.getStr().equals("2")).flatMap( sc -> sc.getAl());   

To

ArrayList<String> als = sca.get(0).getAl();

Answer 3

First you have to use List instead of ArrayList. So with List you code will looks like

List<String> als1 = null;
    List<String> als = sca.stream().
            filter(s -> s.getStr().equals("2")).  //Comparing 
            map(s -> s.getAl())  // Converting List<SampleClass> to list of all al list inside all SampleClass in format List<List<Straing>> 
            .flatMap(ArrayList::stream)  //Creating a flat list from list of list of List ::  List<List<Straing>> --To--> List<String>
            .collect(Collectors.toList());  // Collecting as list

I have commented this code with details. But here if there are two SampleCalss objects in the list with str=2 then it will merge the al list of both objects. hope it will help you .

Answer 4

I'm trying to get the ArrayList stored inside the ArrayList of objects.

Well, the basic algorithm is as follows: Filter sca so it only leaves elements where str is "2" -> Get a single element from all the left over elements -> Get the al stored inside of that element.

You have done the first part correctly:

sca.stream().filter( s -> s.getStr().equals("2"))

But now you need to get a single element from the filtered result (filter can result in multiple elements being left over), so you call findFirst :

.findFirst().get()

This get call will throw an exception if there is no element left after the filter. If you don't want it to throw an exception, you can replace it with an orElse call:

.findFirst.orElse(new SampleClass("", null))

If you use orElse , the method chain will evaluate to null if no element with str being "2".

Now you just need to get the array list by calling getAl() :

.getAl();

Now we combine all this together:

ArrayList<String> als = sca.stream()
    .filter( s -> s.getStr().equals("2"))
    .findFirst().orElse(new SampleClass("", null)).getAl();