Java code to bytecode to constant pool hashes

From Java Code → Bytecode → Constant Pool Hashes

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Code Example 🎯

Let’s take the simplest code:

Java Code

public class Demo {
    public static void main(String[] args) {
        Demo d = new Demo();
        String msg = d.sayHello();
        System.out.println(msg);
    }

    public String sayHello() {
        return "Hello World";
    }
}

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Step 1️⃣

Bytecode Generated 🔥

public static void main(java.lang.String[]);
   0: new           #2          // Create Object Demo
   3: dup                       // Duplicate Reference
   4: invokespecial #1          // Call Constructor
   7: astore_1                  // Store Reference in Local Variable
   8: aload_1                   // Load Reference
   9: invokevirtual #3          // Call sayHello()
  12: astore_2                  // Store Result in Variable
  13: getstatic     #4          // Get System.out
  16: aload_2                   // Load Result
  17: invokevirtual #5          // Call println()
  20: return

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Step 2️⃣

What JVM Sees 🔥

Let’s break this down:

Bytecode	Meaning	Hash
new #2	Create Object	#2 = Demo Class
dup	Duplicate Reference	–
invokespecial #1	Call Constructor	#1 = Demo.<init>()
astore_1	Store Reference in Local Variable	–
aload_1	Load Reference	–
invokevirtual #3	Call Method	#3 = Demo.sayHello()
astore_2	Store Result	–
getstatic #4	Get System.out	#4 = System.out
aload_2	Load Result	–
invokevirtual #5	Call println()	#5 = PrintStream.println()
return	Method End	–

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Step 3️⃣

What are These Hashes?

These hashes represent Constant Pool Table 💪

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Constant Pool Table

Hash	Type	What it Points To
#1	Methodref	Demo.<init>()V (Constructor)
#2	Class	Demo Class
#3	Methodref	Demo.sayHello()Ljava/lang/String;
#4	Fieldref	System.out
#5	Methodref	PrintStream.println(Ljava/lang/String;)V

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Now Let’s Connect Everything 🔥

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Line 1

javaCopyEditDemo d = new Demo();

Bytecode:

0: new #2                  // Create Object Demo
3: dup                     // Duplicate Reference
4: invokespecial #1        // Call Constructor
7: astore_1                // Store Reference

✅ What Happens:

Opcode	Hash	Meaning
new #2	#2	Create Object Demo
invokespecial #1	#1	Call Constructor Demo.<init>()
astore_1	–	Store Reference in Local Variable

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Line 2

String msg = d.sayHello();

Bytecode:

asmCopyEdit8: aload_1                 // Load Reference

Line 2️⃣

javaCopyEditString msg = d.sayHello();

Bytecode:

8: aload_1                   // Load reference 'd'
9: invokevirtual #3           // Call sayHello()
12: astore_2                  // Store result in variable 'msg'

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What happens here?

Opcode	Hash	Meaning
aload_1	–	Load reference d
invokevirtual #3	#3	Call method sayHello() from the object
astore_2	–	Store the result in msg

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How JVM Resolves this?

👉 invokevirtual #3 checks the Constant Pool Table

Constant Pool Entry for #3

#3 = Methodref Demo.sayHello()Ljava/lang/String;

✅ This means:

• Class = Demo
• Method = sayHello()
• Return Type = String

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Line 3️⃣

javaCopyEditSystem.out.println(msg);

Bytecode:

13: getstatic     #4                 // Get System.out
16: aload_2                         // Load result (msg)
17: invokevirtual #5                 // Call println()
20: return

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What happens here?

Opcode	Hash	Meaning
getstatic #4	#4	Get System.out
aload_2	–	Load msg
invokevirtual #5	#5	Call println() method

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Constant Pool Entry for #4

#4 = Fieldref java/lang/System.out Ljava/io/PrintStream;

✅ This means:

• Class = System
• Field = out
• Type = PrintStream

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Constant Pool Entry for #5

#5 = Methodref java/io/PrintStream.println(Ljava/lang/String;)V

✅ This means:

• Class = PrintStream
• Method = println
• Argument = String
• Return = void

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Now Bhai 🧠💪

Let’s connect ALL HASHES 🔥🔥

Bytecode	Hash	Constant Pool Entry
new #2	#2	Class -> Demo
invokespecial #1	#1	Method -> Demo.<init>()
invokevirtual #3	#3	Method -> Demo.sayHello()
getstatic #4	#4	Field -> System.out
invokevirtual #5	#5	Method -> PrintStream.println()

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💥 Final Picture

Demo d = new Demo();        → new #2 → invokespecial #1
String msg = d.sayHello();   → invokevirtual #3
System.out.println(msg);     → getstatic #4 → invokevirtual #5

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Now Coming to Your Million Dollar Question 💰💪

🔥 Why This Constant Pool Exists?

✅ Because in JVM, bytecode must be small and compact
Instead of storing names like "sayHello" or "println" everywhere,
it stores just the HASH Reference Numbers.

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