java - How to Summarize record of Array-list of Object -

please following problem.

    //bean class public class test {      private int value;     private int com_id;      /**      * @return value      */     public int getvalue() {         return value;     }      /**      * @param value value set      */     public void setvalue(int value) {         this.value = value;     }      /**      * @return com_id      */     public int getcom_id() {         return com_id;     }      /**      * @param com_id com_id set      */     public void setcom_id(int com_id) {         this.com_id = com_id;     }  } 

main class

public class test2 {      public static void main(string[] args) {         test bean1 = new test();         test bean2 = new test();         test bean3 = new test();         test bean4 = new test();         test bean5 = new test();          //bean1         bean1.setcom_id(1);         bean1.setvalue(10);         //bean2          bean2.setcom_id(2);         bean2.setvalue(50);         //bean3         bean3.setcom_id(1);         bean3.setvalue(30);         //bean4          bean4.setcom_id(1);         bean4.setvalue(20);         //bean5         bean5.setcom_id(2);         bean5.setvalue(20);          //arraylist of test object         arraylist<test> alltest = new arraylist<test>();         alltest.add(bean1);         alltest.add(bean2);         alltest.add(bean3);         alltest.add(bean4);         alltest.add(bean5);      }  } 

now want arraylist sum of value com_id same follow answer_list com_id = 1; val = 60 (10+30+20) com_id = 2; val = 70 (50+20)

you can old way loops, or in 1 (very long) line using java 8 streams:

list<test> aggregated =      alltest.stream()            .collect(collectors.groupingby(test::getcom_id,                                           collectors.summingint(test::getvalue)))            .entryset()            .stream()            .map(e-> new test(e->getkey(),e->getvalue())            .collect(collectors.tolist()); 

explanation :

the first part (until end of first collect) should create map<integer,integer>, key test identifier , value sum of values of test instances having same id.

the second part iterates on entries of map , tranforms list of test instances.

note requires adding constructor test class taking 2 parameters.

public test (int com_id, int value) {     this.com_id = com_id;     this.value = value; } 

this not tested, may contain typos.