How to install:

1. Download .jar file
2. In intellij idea go to File -> Settings
3. Go to Plugins
4. Click to settings icon near "Marketplace" and "Installed"
5. Click "Install Plugin from disk" and choose .jar
6. Restart IDE

Checking rules for now:

1. Returning 'null' in method:

public Integer calc(int a, int b) {
    if(a < 0 || b < 0) {
        return null;    
    }
    // calc
} 

must be:

public Integer calc(int a, int b) {
    if(a < 0 || b < 0) {
        // or throw exception
        // or new YourEntity extends Number with constructor (int val)
        return 0;
    }
    // calc    
} 

2. Method has to have only one 'return' statement!

(There might be better example)

public boolean greatherThan(int a) {
    if(a > 0) {
        return true;
    }
    return false;
}

must be:

public boolean greatherThan(int a) {
    final boolean result;
    if(a > 0) {
        result = true;
    } else {
        result = false;    
    }
    return result;
}

3. Checking for all final class & method variables for immutability.

Also checking when for(int i = 0;) i may be replaced by objects iteration