how to use the sum method in java

I am reading in a text file with four columns of data to a HashMap. I want to sum the value columns. Can I get an开发者_JAVA技巧 example on using the sum method?

There is no such method like that. There's however the additive operator + which can be used on numeric primitives/types like int and Integer.

Assuming that you've a Map<String, Integer>, here's an example:

int total = 0;
for (Integer value : map.values()) {
    total = total + value; // Can also be done by total += value;
}
System.out.println(total); // Should print the total.

See also:

• Java tutorial - operators
• Java tutorial - the Map interface
• Java tutorial - the for statement

Update: I just wanted to add one other hint; your core problem might be that you've the numbers in flavor of String objects (since you're parsing a text file) and the + of course won't sum them up, but just concatenate them. You'd like to convert each number from String to Integer first. This can be done with Integer#valueOf(). E.g.

String numberAsString = "10";
Integer numberAsInteger = Integer.valueOf(numberAsString);
// Now put in map and so on.

This way you can do basic arithmetic with numbers as intended.

Based on your question, I made the assumption that you actually need to know how to merge three value columns into one value so you can put it into a map. Here's an example which converts value columns of any type into one string and then puts it into a map. It assumes the first column is the key column and makes no assumptions on their data types.

//These are values for each column. column1 to column4.
Object column1 = ...
Object column2 = ...
Object column3 = ...
Object column4 = ...

//This is the map you are reading the values into.
Map<Object, String> map = new HashMap<Object, String>();

StringBuilder sb = new StringBuilder();
sb.append(column2);
sb.append(column3);
sb.append(column4);

map.put(column1, sb.toString());