How to change the local variable of the outer method in the lambdas foreach block?

问题

Not for-each block:

public void cleanup4(){
    long timestamp = Instant.now().minus(90l, ChronoUnit.DAYS)
            .toEpochMilli();
    String cql = "SELECT * FROM usertoken";
    ResultSet rs = CassandraTools.getInstance().execute(cql,ConsistencyLevel.QUORUM);
    CassandraConverter cassandraConverter=new MappingCassandraConverter();
    CassandraConverterRowCallback<UserTokenCassandra> callback= new CassandraConverterRowCallback<UserTokenCassandra>(cassandraConverter, UserTokenCassandra.class);
    UserTokenCassandra utc = null;
    Iterator<Row> iterator = rs.iterator();
    while (iterator.hasNext()) {
        Row row = iterator.next();
        utc=callback.doWith(row);
        log.info("{}", utc);
        log.info("记录的时间为：{}",
                Instant.ofEpochMilli(utc.getExpireTime()));
        if (timestamp > utc.getExpireTime()) {
            // repository.delete(n);
        }
    }
}

For-each block:

public void cleanup4(){
    long timestamp = Instant.now().minus(90l, ChronoUnit.DAYS)
            .toEpochMilli();
    String cql = "SELECT * FROM usertoken";
    ResultSet rs = CassandraTools.getInstance().execute(cql,ConsistencyLevel.QUORUM);
    CassandraConverter cassandraConverter=new MappingCassandraConverter();
    CassandraConverterRowCallback<UserTokenCassandra> callback= new CassandraConverterRowCallback<UserTokenCassandra>(cassandraConverter, UserTokenCassandra.class);
    rs.forEach(usertoken->{
        UserTokenCassandra utc = null;
        log.info("{}", utc);
        log.info("The Time：{}",
                Instant.ofEpochMilli(utc.getExpireTime()));
        if (timestamp > utc.getExpireTime()) {
            // repository.delete(utc);
        }
    });
}

1.The for-each block seems generate a lot no use reference of UserTokenCassandra .. And are these garbage? 2.So How to write this code?

回答1:

You cannot modify the outer method local variable from within the lambda body. Here you don't need the forEach. As ResultSet implements Iterable, you can use enhanced for loop instead:

for(Row row : rs) {
    utc=callback.doWith(row);
    log.info("{}", utc);
    log.info("记录的时间为：{}",
            Instant.ofEpochMilli(utc.getExpireTime()));
    if (timestamp > utc.getExpireTime()) {
        // repository.delete(n);
    }
}

来源：https://stackoverflow.com/questions/32561029/how-to-change-the-local-variable-of-the-outer-method-in-the-lambdas-foreach-bloc