[Java] Try to implement using generics

Here are some notes on implementations using Generics.

Takes Class \ <T > and returns an object with generics \ <T >

If you declare something that specifies generics normally, it tends to be redundant, so if you want to generate such an object many times, I think it is better to create a generation function in the Util class.

Implementation example

//A function that returns a List corresponding to the class
<T> List<T> generateList(Class<T> clazz) {
    return new ArrayList<>();

//A function that returns a BeanPropertyRowMapper that corresponds to the class
<T> BeanPropertyRowMapper<T> getBeanPropertyRowMapper(Class<T> clazz) {
    return new BeanPropertyRowMapper<>(clazz);

Usage example

//Redundant when written like this
new BeanPropertyRowMapper<Something>(Something.class);

//It is easier to read if you write like this

Write so that no warning / error is issued when the type does not matter

In the case of "use a type that takes generics, but the type of the contents does not matter", simply omitting the type will give a warning.

Example of warning

//Duplicate check using hash set
boolean checkForDuplicate(List list) {
    return list.size() == new HashSet(list).size();

In such cases, you can write <?> And the diamond operator to avoid warnings.

Implementation example

//Duplicate check using hash set
boolean checkForDuplicate(List<?> list) {
    return list.size() == new HashSet<>(list).size();

You can write it by specifying it with <T>, but it will be a little redundant.

Redundant implementation example

//Duplicate check using hash set
<T> boolean checkForDuplicate(List<T> list) {
    return list.size() == new HashSet<>(list).size();

When using Stream

Even when performing Stream processing with List as an argument, compilation may not pass unless <?> Is written.

Example that compilation does not pass

BeanPropertySqlParameterSource[] makeParamArray(List entities) {
    return entities.stream()

Example of compiling

BeanPropertySqlParameterSource[] makeParamArray(List<?> entities) {
    return entities.stream()

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