Understanding Solidity Constructors: From Java Basics to Version‑Specific Pitfalls

In object‑oriented languages like Java each class can have one or more constructors, and Solidity provides a similar feature.

Java Constructors

If a Java class does not declare a constructor, the compiler supplies a default one.

public class DemoTest {
}

When a constructor is declared, object creation must use the declared form:

public class DemoTest {
    public DemoTest() {
    }
}

Solidity Constructors (Pre‑0.4.22)

Older Solidity versions (< 0.4.22) used the contract name as a function to define a constructor.

pragma solidity ^0.4.21;
contract DemoTest {
}

Explicit constructor using a function:

pragma solidity ^0.4.21;
contract DemoTest {
    function DemoTest() public {
    }
}

Version‑Induced Deprecation

Compiling the above with Solidity 0.4.22 or later produces a warning:

Warning: Defining constructors as functions with the same name as the contract is deprecated. Use "constructor(...) { ... }" instead.

From version 0.4.22 onward, the dedicated constructor keyword must be used.

Solidity Constructors (≥ 0.4.22)

pragma solidity ^0.4.22;
contract DemoTest {
    constructor() public {
    }
}

If the newer syntax is compiled with an older compiler (e.g., 0.4.21), a parser error occurs:

ParserError: Expected identifier, got 'LParen'

These mismatches are commonly seen when using development tools such as Remix or Truffle.

Summary

The article outlines how Solidity constructors evolved, provides side‑by‑side Java comparisons, shows the exact syntax for each Solidity version, and explains the warnings and errors that arise when the contract code and compiler version are inconsistent.