Modern Map methods: getOrDefault, merge, computeIfAbsent

What you will learn

• Read with a fallback using getOrDefault
• Create or update one entry with computeIfAbsent, computeIfPresent and compute
• Combine old and new values with merge, and remove an entry by returning null
• Change every value with replaceAll and guard a change with replace(key, old, new)
• Choose the right Map method for a job and avoid changing the map inside its lambda

Topics in this lesson

1. getOrDefault: read with a fallback
2. computeIfAbsent and computeIfPresent
3. compute and merge: change a value
4. replaceAll, replace and remove with a value
5. Which method for which job?

Key points

• getOrDefault reads with a fallback and never changes the map.
• computeIfAbsent makes a value only when needed; computeIfPresent changes only an existing one; compute always runs.
• merge(k, v, f) stores v for a new key, else f(old, v); a null result from compute or merge removes the entry.
• Never change the same map inside the lambda; read, calculate, then put.

Interview questions this lesson answers

• What is the difference between getOrDefault and computeIfAbsent?
• What do compute, computeIfAbsent and computeIfPresent do?
• What happens when the lambda of compute or merge returns null?
• When would you choose merge instead of compute?
• Why should the lambda of computeIfAbsent not change the same map?
• Are these methods thread-safe?
• Write a one-line page-visit counter.