Java Conversions: Understanding valueOf(), parseInt(), and toString()

For years, I found myself second-guessing which method to use when converting between primitives, wrapper classes, and strings in Java. Do I use valueOf() here? What about parseInt()? When should I use toString()?

If that confusion sounds familiar, hopefully this guide will clean it up.

🔹 Converting to a Wrapper Class? → Use valueOf()

The first thing to know is: valueOf() is not some universal magic converter.

It works only when you're converting to a wrapper of a specific type, like Integer, Double, Boolean, etc. And it accepts:

• a primitive of the same type (e.g., int for Integer)

• or a String that represents a value of that type (e.g., "123" for Integer)

Examples:

javaCopyEditInteger i1 = Integer.valueOf(100);       // primitive int → Integer
Integer i2 = Integer.valueOf("100");     // String → Integer
Boolean b  = Boolean.valueOf("true");    // String → Boolean

This is where most people stop and assume valueOf() works for converting anything into anything — but that's wrong.

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🔸 Converting One Wrapper to Another? → You Must Unbox and Rebox

Let’s say you want to convert an Integer to a Double.
Can you do this?

javaCopyEditInteger i = 10;
Double d = Double.valueOf(i);   // ❌ Won’t compile

Nope. Double.valueOf() does not accept an Integer — it expects a double or a String.

✅ Correct Way:

javaCopyEditDouble d = Double.valueOf(i.doubleValue()); // Integer → double → Double

This is key: Java doesn't auto-convert between different wrapper types. You have to extract the primitive, convert it if needed, and then wrap it again.

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🔹 Converting to a Primitive? → Use parseXxx() (Always)

If your end goal is a primitive (int, double, boolean, etc.), the answer is always the parseXxx() methods:

javaCopyEditint i = Integer.parseInt("123");           // String → int
double d = Double.parseDouble("45.67");    // String → double
boolean b = Boolean.parseBoolean("true");  // String → boolean

That’s it. Use parse when you want to strip the object and get a raw value.
Don't overthink it.

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🔹 Converting to a String?

This one also gets unnecessarily confusing. Here's the rule:

👉 If you’re starting from a primitive — use String.valueOf(...):

javaCopyEditString s = String.valueOf(123);         // int → String
String s2 = String.valueOf(false);      // boolean → String

👉 If you're starting from a wrapper — you can:

javaCopyEditString s = Integer.valueOf(999).toString();   // Wrapper → String

Or if you want to keep things generic:

javaCopyEditString s = String.valueOf(Integer.valueOf(888)); // Still works