Java 9 private methods in Interface

Java 9 supports Java 9 private methods in Interface to enable code sharing between non-abstract methods.

Consider java 8 example where we want to print market two most popular search engines Solr and elastic search features.

Example 1 : Java 8 Support Default Method in interface

public interface ISearchEngine {
    
    default void printSolrFeatures(){
        System.out.println("Http Rest Api");
        System.out.println("ID field for updates and deduplication");
        System.out.println("DocValues");
        
        System.out.println("Query-time synonyms");
        System.out.println("Master-slave replication");
        System.out.println("Data import:DataImportHandler - JDBC, CSV, XML, Tika, URL, Flat File");
    }
    default void printElasticSearchFeatures(){
        System.out.println("Http Rest Api");
        System.out.println("ID field for updates and deduplication");
        System.out.println("DocValues");
        
        System.out.println("Multiple document types per schema");
        System.out.println("Data import:Rivers modules - ActiveMQ, Amazon SQS, CouchDB, Dropbox, " +
                "DynamoDB, FileSystem, Git, GitHub, Hazelcast, JDBC");
    }
}

As we see in above example most of the features are same in both the search engine.It’s redundancy of code.

To avoid such problems java 9 interface provide handly features called private methods.

Example 2: Java 9 private methods in interface

public interface ISearchEngine {
    private void printCommonFeatures(){                // java 9 allowed to create private methods, It can not be accessible out site of interface
        System.out.println("Http Rest Api");
        System.out.println("ID field for updates and deduplication");
        System.out.println("DocValues");
    }
    default void printSolrFeatures(){
        printCommonFeatures();
        System.out.println("Query-time synonyms");
        System.out.println("Master-slave replication");
        System.out.println("Data import:DataImportHandler - JDBC, CSV, XML, Tika, URL, Flat File");
    }
    default void printElasticSearchFeatures(){
        printCommonFeatures();
        System.out.println("Multiple document types per schema");
        System.out.println("Data import:Rivers modules - ActiveMQ, Amazon SQS, CouchDB, Dropbox, " +
                "DynamoDB, FileSystem, Git, GitHub, Hazelcast, JDBC");
    }
}

Great! we can write private method using private access modifiers.

Output:

************ Solr Features *************
Http Rest Api
ID field for updates and deduplication
DocValues
Query-time synonyms
Master-slave replication
Data import:DataImportHandler - JDBC, CSV, XML, Tika, URL, Flat File
************ Elastic Search Features *************
Http Rest Api
ID field for updates and deduplication
DocValues
Multiple document types per schema
Data import:Rivers modules - ActiveMQ, Amazon SQS, CouchDB, Dropbox, DynamoDB, FileSystem, Git, GitHub, Hazelcast, JDBC

java 9 and later release interface can have following:

1. Constant variables
2. Abstract methods
3. Default methods
4. Static methods
5. Private methods
6. Private static methods