public static void main(String[] args) { 
        String matcherStr = "This is the first Java";
        matcherStr += "\nAnd";
        matcherStr += "\nThis is the second Python";
        Matcher matcher = Pattern.compile("Java$", Pattern.MULTILINE).matcher(matcherStr);
        
        int i=0;
        while(matcher.find()){
           i++;
        }
        System.out.println(i);
}

There is one match (i=1) for Java in the first line. This is mutiline, so the whole matcherStr is something like:

This is the first Java

And

This is the second Python

public static void main(String[] args) { 
        String matcherStr = "This is the first Java";
        matcherStr += "\nAnd";
        matcherStr += "\nThis is the second Java";
        Matcher matcher = Pattern.compile("Java$", Pattern.MULTILINE).matcher(matcherStr);
        
        int i=0;
        while(matcher.find()){
           i++;
        }
        System.out.println(i);
}

There are two matchs (i=2) for Java in the first line. This is mutiline, so the whole matcherStr is something like:

This is the first Java

And

This is the second Java

public static void main(String[] args) { 
        String matcherStr = "This is the first Java";
        matcherStr += "\nAnd";
        matcherStr += "\nThis is the second Python";
        Matcher matcher = Pattern.compile("Java$").matcher(matcherStr);
        
        int i=0;
        while(matcher.find()){
           i++;
        }
        System.out.println(i);
}

There is no match (i=0) for Java. This is not mutiline, so the whole matcherStr is something like:

This is the first Java\nAnd\nThis is the second Python

public static void main(String[] args) { 
        String matcherStr = "This is the first Java";
        matcherStr += "\nAnd";
        matcherStr += "\nThis is the second Java";
        Matcher matcher = Pattern.compile("Java\\Z", Pattern.MULTILINE).matcher(matcherStr);
        
        int i=0;
        while(matcher.find()){
           i++;
        }
        System.out.println(i);
}

There is one match (i=1) for Java with multiline.  The same as without multiline.

In this case: If Pattern.compile("Java$", Pattern.MULTILINE), there are two matches.

public static void main(String[] args) { 
        String matcherStr = "This is the first Java";
        matcherStr += "\nAnd";
        matcherStr += "\nThis is the second Java";
        Matcher matcher = Pattern.compile("Java\\Z").matcher(matcherStr);
        
        int i=0;
        while(matcher.find()){
           i++;
        }
        System.out.println(i);
}

There is one match (i=1) for Java without multiline.