What is G1 String Deduplication?

How it works:

• During GC, G1 scans young-gen Strings.
• If two Strings have the same content, both point to one shared byte[] array. The duplicate array is freed.
• The String objects remain separate – only the internal array is shared.
• No code changes needed. Just a JVM flag.

How to enable

java -XX:+UseStringDeduplication -XX:+UseG1GC -jar bank-app.jar

How to measure savings

-XX:+PrintStringDeduplicationStatistics
# Shows: strings inspected, deduplicated, bytes saved

String.intern() vs G1 dedup

| Feature | String.intern() | G1 String Dedup |
|---|---|---|
| What is shared | The String object | The byte[] array |
| How | Manual call | Automatic during GC |
| Code change | Yes | No (JVM flag) |
| Thread safety | Native hash table contention | GC-integrated |
| Best for | Known repeated values | Unknown/automatic |

What should a senior engineer know?

• CPU overhead: Dedup adds ~2-5% GC overhead for scanning and comparing.
• Works only on young-gen strings (they must survive a few GC cycles to be candidates).
• ZGC and Shenandoah do not support string dedup as of JDK 21.

What are common mistakes?

A common mistake is not understanding the concept fully before using it in production banking applications.

Wrong approach:

package com.bank.service;

public class TransferService {
    public void transfer(String fromAccount, String toAccount, double amount) {
        // WRONG: using double for money
        double balance = getBalance(fromAccount);
        balance = balance - amount;
    }
    private double getBalance(String id) { return 10000.0; }
}

Correct approach:

package com.bank.service;

import java.math.BigDecimal;

public class TransferService {
    public void transfer(String fromAccount, String toAccount, BigDecimal amount) {
        // CORRECT: using BigDecimal for money
        BigDecimal balance = getBalance(fromAccount);
        balance = balance.subtract(amount);
    }
    private BigDecimal getBalance(String id) { return new BigDecimal("10000.00"); }
}

Always use BigDecimal for monetary calculations in banking systems to avoid floating-point precision errors.

What are the edge cases?

Edge cases can cause unexpected behavior in banking applications. Here are some to watch for.

package com.bank.edge;

public class EdgeCaseDemo {
    public static void main(String[] args) {
        // Edge case 1: null handling
        String accountId = null;
        try {
            System.out.println(accountId.length());
        } catch (NullPointerException e) {
            System.out.println("Null account ID");
        }

        // Edge case 2: empty vs null
        String emptyId = "";
        System.out.println(emptyId.isEmpty());  // true
        System.out.println(emptyId == null);     // false

        // Edge case 3: boundary values
        java.math.BigDecimal zero = java.math.BigDecimal.ZERO;
        java.math.BigDecimal negativeBalance = new java.math.BigDecimal("-0.01");
        System.out.println(zero.compareTo(negativeBalance) > 0);  // true
    }
}

Null account ID
true
false
true

Always test with null values, empty strings, boundary values, and negative amounts in banking applications.

What are common interview follow-up questions?

Interviewers often follow up with these related questions:

• How does this concept apply to a high-throughput banking transaction system?
• What are the performance implications in a production environment?
• How would you debug issues related to this concept?
• What tools would you use to monitor this in production?
• How does this behave differently in Java 8 vs Java 17+?

When answering follow-up questions, always relate your answer back to practical experience. For example, at National Bank of India, Rajesh debugged a production issue related to this concept by using JFR recordings and heap dumps. Providing concrete examples from banking systems shows the interviewer that you understand both the theory and the practice.