Compare two lists of different types using HashMap

I have two classes:

public class AClass{



    String name;

    int id;

    int total;

}



public class BClass{

    String batchName;

    int id;

}

Now I have two lists:

List<AClass> aLst;

List<BClass> bLst;

Between these two list i need to check if AClass.id==BClass.id.
One way to achieve this is to have two for loops and compare. But this is not the efficient way.

Other way is by using HashMap. Traverse the BClass list and use the BClass.id as the key and the respective object as the value:

Map<int,BClass> map = new HashMap<int,BClass>();

List requiredLst <BClass> = new ArrayList<BClass>();

foreach(BClass b : bLst){

    map.put(BClass.id, b);

}



foreach(AClass a : aLst){

    BClass b = map.get(a.id);

    requiredLst.add(b);

}

Is this efficient way to compare?

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

asked Nov 19 '18 at 19:30

James Patty

111

You haven't sufficiently described the actual comparison procedure. What does the procedure return, a boolean? What if the 5th element in the first list has the same id as the 13th element in the second list?

– amn
Nov 19 '18 at 19:34

Are you looking just for missing ids or for entire objects?

– Karol Dowbecki
Nov 19 '18 at 19:52

I am looking for all the objects of BClass in bLst having the same id's as that of the ids in the AClass objects ..

– James Patty
Nov 19 '18 at 20:36

We can safely assume that IDs are unquie...

– James Patty
Nov 19 '18 at 20:37

add a comment |

I have two classes:

public class AClass{



    String name;

    int id;

    int total;

}



public class BClass{

    String batchName;

    int id;

}

Now I have two lists:

List<AClass> aLst;

List<BClass> bLst;

Between these two list i need to check if AClass.id==BClass.id.
One way to achieve this is to have two for loops and compare. But this is not the efficient way.

Other way is by using HashMap. Traverse the BClass list and use the BClass.id as the key and the respective object as the value:

Map<int,BClass> map = new HashMap<int,BClass>();

List requiredLst <BClass> = new ArrayList<BClass>();

foreach(BClass b : bLst){

    map.put(BClass.id, b);

}



foreach(AClass a : aLst){

    BClass b = map.get(a.id);

    requiredLst.add(b);

}

Is this efficient way to compare?

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

asked Nov 19 '18 at 19:30

James Patty

111

You haven't sufficiently described the actual comparison procedure. What does the procedure return, a boolean? What if the 5th element in the first list has the same id as the 13th element in the second list?

– amn
Nov 19 '18 at 19:34

Are you looking just for missing ids or for entire objects?

– Karol Dowbecki
Nov 19 '18 at 19:52

I am looking for all the objects of BClass in bLst having the same id's as that of the ids in the AClass objects ..

– James Patty
Nov 19 '18 at 20:36

We can safely assume that IDs are unquie...

– James Patty
Nov 19 '18 at 20:37

add a comment |

I have two classes:

public class AClass{



    String name;

    int id;

    int total;

}



public class BClass{

    String batchName;

    int id;

}

Now I have two lists:

List<AClass> aLst;

List<BClass> bLst;

Between these two list i need to check if AClass.id==BClass.id.
One way to achieve this is to have two for loops and compare. But this is not the efficient way.

Other way is by using HashMap. Traverse the BClass list and use the BClass.id as the key and the respective object as the value:

Map<int,BClass> map = new HashMap<int,BClass>();

List requiredLst <BClass> = new ArrayList<BClass>();

foreach(BClass b : bLst){

    map.put(BClass.id, b);

}



foreach(AClass a : aLst){

    BClass b = map.get(a.id);

    requiredLst.add(b);

}

Is this efficient way to compare?

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

asked Nov 19 '18 at 19:30

James Patty

111

I have two classes:

public class AClass{



    String name;

    int id;

    int total;

}



public class BClass{

    String batchName;

    int id;

}

Now I have two lists:

List<AClass> aLst;

List<BClass> bLst;

Between these two list i need to check if AClass.id==BClass.id.
One way to achieve this is to have two for loops and compare. But this is not the efficient way.

Other way is by using HashMap. Traverse the BClass list and use the BClass.id as the key and the respective object as the value:

Map<int,BClass> map = new HashMap<int,BClass>();

List requiredLst <BClass> = new ArrayList<BClass>();

foreach(BClass b : bLst){

    map.put(BClass.id, b);

}



foreach(AClass a : aLst){

    BClass b = map.get(a.id);

    requiredLst.add(b);

}

Is this efficient way to compare?

java arraylist collections hashmap processing-efficiency

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

asked Nov 19 '18 at 19:30

James Patty

111

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

asked Nov 19 '18 at 19:30

James Patty

111

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

edited Nov 19 '18 at 21:13

Karol Dowbecki

21k93054

asked Nov 19 '18 at 19:30

James Patty

111

asked Nov 19 '18 at 19:30

James Patty

111

asked Nov 19 '18 at 19:30

James Patty

111

You haven't sufficiently described the actual comparison procedure. What does the procedure return, a boolean? What if the 5th element in the first list has the same id as the 13th element in the second list?

– amn
Nov 19 '18 at 19:34

Are you looking just for missing ids or for entire objects?

– Karol Dowbecki
Nov 19 '18 at 19:52

I am looking for all the objects of BClass in bLst having the same id's as that of the ids in the AClass objects ..

– James Patty
Nov 19 '18 at 20:36

We can safely assume that IDs are unquie...

– James Patty
Nov 19 '18 at 20:37

add a comment |

You haven't sufficiently described the actual comparison procedure. What does the procedure return, a boolean? What if the 5th element in the first list has the same id as the 13th element in the second list?

– amn
Nov 19 '18 at 19:34

Are you looking just for missing ids or for entire objects?

– Karol Dowbecki
Nov 19 '18 at 19:52

I am looking for all the objects of BClass in bLst having the same id's as that of the ids in the AClass objects ..

– James Patty
Nov 19 '18 at 20:36

We can safely assume that IDs are unquie...

– James Patty
Nov 19 '18 at 20:37

You haven't sufficiently described the actual comparison procedure. What does the procedure return, a boolean? What if the 5th element in the first list has the same id as the 13th element in the second list?

– amn
Nov 19 '18 at 19:34

Are you looking just for missing ids or for entire objects?

– Karol Dowbecki
Nov 19 '18 at 19:52

I am looking for all the objects of BClass in bLst having the same id's as that of the ids in the AClass objects ..

– James Patty
Nov 19 '18 at 20:36

We can safely assume that IDs are unquie...

– James Patty
Nov 19 '18 at 20:37

add a comment |

1 Answer
1

active

oldest

votes

It would be easier with a Set<Integer> to store the identifiers from AClass. Since Set.contains() is O(1) the total complexity will be O(n).

List<AClass> aLst = ...;

List<BClass> bLst = ...;



Set<Integer> ids = aLst.stream()

    .map(AClass::getId)

    .collect(Collectors.toSet());

List<BClass> required = bLst.stream()

    .filter(b -> ids.contains(b.getId()))

    .collect(Collectors.toList());

However for small lists the cost of creating additional Set and boxing identifiers into Integer might outweigh the benefit of reduced number of iterations.

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

It would be easier with a Set<Integer> to store the identifiers from AClass. Since Set.contains() is O(1) the total complexity will be O(n).

List<AClass> aLst = ...;

List<BClass> bLst = ...;



Set<Integer> ids = aLst.stream()

    .map(AClass::getId)

    .collect(Collectors.toSet());

List<BClass> required = bLst.stream()

    .filter(b -> ids.contains(b.getId()))

    .collect(Collectors.toList());

However for small lists the cost of creating additional Set and boxing identifiers into Integer might outweigh the benefit of reduced number of iterations.

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

add a comment |

It would be easier with a Set<Integer> to store the identifiers from AClass. Since Set.contains() is O(1) the total complexity will be O(n).

List<AClass> aLst = ...;

List<BClass> bLst = ...;



Set<Integer> ids = aLst.stream()

    .map(AClass::getId)

    .collect(Collectors.toSet());

List<BClass> required = bLst.stream()

    .filter(b -> ids.contains(b.getId()))

    .collect(Collectors.toList());

However for small lists the cost of creating additional Set and boxing identifiers into Integer might outweigh the benefit of reduced number of iterations.

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

add a comment |

It would be easier with a Set<Integer> to store the identifiers from AClass. Since Set.contains() is O(1) the total complexity will be O(n).

List<AClass> aLst = ...;

List<BClass> bLst = ...;



Set<Integer> ids = aLst.stream()

    .map(AClass::getId)

    .collect(Collectors.toSet());

List<BClass> required = bLst.stream()

    .filter(b -> ids.contains(b.getId()))

    .collect(Collectors.toList());

However for small lists the cost of creating additional Set and boxing identifiers into Integer might outweigh the benefit of reduced number of iterations.

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

It would be easier with a Set<Integer> to store the identifiers from AClass. Since Set.contains() is O(1) the total complexity will be O(n).

List<AClass> aLst = ...;

List<BClass> bLst = ...;



Set<Integer> ids = aLst.stream()

    .map(AClass::getId)

    .collect(Collectors.toSet());

List<BClass> required = bLst.stream()

    .filter(b -> ids.contains(b.getId()))

    .collect(Collectors.toList());

However for small lists the cost of creating additional Set and boxing identifiers into Integer might outweigh the benefit of reduced number of iterations.

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

answered Nov 19 '18 at 20:06

Karol Dowbecki

21k93054

add a comment |

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