Grouping a list of main objects by their nested object's id
Given the following class A :
public class A { private int id; //this field is unique private int a_1; //this field ain't unique private String a_2; private String a_3; //setters + getters } We can group a list of random objects of type A as follow by their respective a_1 :
Map<String, List<A>> sortedmap = list.stream() .collect(Collectors.groupingBy(A::getA_1, Collectors.toList())); Now given two classes B and C such as :
public class B{ private int id; //this field is unique for all entities of type B private String b_1; private C b_C; //many objects of type B may have a reference to the same object of type C //setters + getters } public class C{ private int id; //this field is unique for all entities of type C private String c_1; private D c_D; //many objects of type C may have a reference to the same object of type D //setters + getters } public class D{ private int id; //this field is unique for all entities of type D private String d_1; private String d_2; //setters + getters } How can we sort a list of random objects of type B by their respective b_C.getC_D().getId() field?
You can group the results in a TreeMap which is ordered by default:
public static void main(String[] args) { D d1 = new D();d1.setId(1); D d2 = new D();d2.setId(2); C c1 = new C();c1.setC_D(d1); C c2 = new C(); c2.setC_D(d2); B b1 = new B(); b1.setB_C(c1); b1.setId(1); B b2 = new B(); b2.setB_C(c2); b2.setId(2); B b3 = new B(); b3.setB_C(c2); b3.setId(3); Map<String, List<B>> result = Arrays.asList(b1,b2,b3) .stream() .collect(Collectors.groupingBy(b -> ""+b.getB_C().getC_D().getId(), TreeMap::new, Collectors.toList())); System.out.println(result); } The result is:
{1=[B [id=1]], 2=[B [id=2], B [id=3]]} PS. @javaistaucheineinsel nice username 😉
I’d add custom getters to support this (not compiled, forgive typos):
public class B{ private int id; //this field is unique for all entities of type B private String b_1; private C b_C; //many objects of type B may have a reference to the same object of type C //setters + getters public int getBCDId() { return b_C.getCDId(); } } public class C{ private int id; //this field is unique for all entities of type C private String c_1; private D c_D; //many objects of type C may have a reference to the same object of type D //setters + getters public int getCDId() { return c_D.getId(); } } Now you can sort and/or group B objects by the value they return from their getBCDId methods. Just like you would do for B’s own attributes.
